[libc++][chrono] Loads leap-seconds.list in tzdb. (#82113)

This implements the loading of the leap-seconds.list file and store its
contents in the tzdb struct.

This adds the required `leap_seconds` member.

The class leap_seconds is fully implemented including its non-member
functions.

Implements parts of:
- P0355 Extending <chrono> to Calendars and Time Zones
- P1614 The Mothership has Landed

Implements:
- P1981 Rename leap to leap_second
- LWG3359 <chrono> leap second support should allow for negative leap
seconds
- LWG3383 §[time.zone.leap.nonmembers] sys_seconds should be replaced
with seconds

25 files changed, 852 insertions, 23 deletions

diff --git a/libcxx/docs/Status/Cxx20Issues.csv b/libcxx/docs/Status/Cxx20Issues.csv
index f0e9c40..db57b15 100644
--- a/libcxx/docs/Status/Cxx20Issues.csv
+++ b/libcxx/docs/Status/Cxx20Issues.csv
@@ -269,7 +269,7 @@
 "`3355 <https://wg21.link/LWG3355>`__","The memory algorithms should support move-only input iterators introduced by P1207","Prague","|Complete|","15.0","|ranges|"
 "`3356 <https://wg21.link/LWG3356>`__","``__cpp_lib_nothrow_convertible``\  should be ``__cpp_lib_is_nothrow_convertible``\ ","Prague","|Complete|","12.0"
 "`3358 <https://wg21.link/LWG3358>`__","|sect|\ [span.cons] is mistaken that ``to_address``\  can throw","Prague","|Complete|","17.0"
-"`3359 <https://wg21.link/LWG3359>`__","``<chrono>``\  leap second support should allow for negative leap seconds","Prague","","","|chrono|"
+"`3359 <https://wg21.link/LWG3359>`__","``<chrono>``\  leap second support should allow for negative leap seconds","Prague","|Complete|","19.0","|chrono|"
 "`3360 <https://wg21.link/LWG3360>`__","``three_way_comparable_with``\  is inconsistent with similar concepts","Prague","|Nothing To Do|","","|spaceship|"
 "`3362 <https://wg21.link/LWG3362>`__","Strike ``stop_source``\ 's ``operator!=``\ ","Prague","",""
 "`3363 <https://wg21.link/LWG3363>`__","``drop_while_view``\  should opt-out of ``sized_range``\ ","Prague","|Nothing To Do|","","|ranges|"
@@ -286,7 +286,7 @@
 "`3380 <https://wg21.link/LWG3380>`__","``common_type``\  and comparison categories","Prague","|Complete|","15.0","|spaceship|"
 "`3381 <https://wg21.link/LWG3381>`__","``begin``\  and ``data``\  must agree for ``contiguous_range``\ ","Prague","|Nothing To Do|","","|ranges|"
 "`3382 <https://wg21.link/LWG3382>`__","NTTP for ``pair``\  and ``array``\ ","Prague","",""
-"`3383 <https://wg21.link/LWG3383>`__","|sect|\ [time.zone.leap.nonmembers] ``sys_seconds``\  should be replaced with ``seconds``\ ","Prague","","","|chrono|"
+"`3383 <https://wg21.link/LWG3383>`__","|sect|\ [time.zone.leap.nonmembers] ``sys_seconds``\  should be replaced with ``seconds``\ ","Prague","|Complete|","19.0","|chrono|"
 "`3384 <https://wg21.link/LWG3384>`__","``transform_view::*sentinel*``\  has an incorrect ``operator-``\ ","Prague","|Complete|","15.0","|ranges|"
 "`3385 <https://wg21.link/LWG3385>`__","``common_iterator``\  is not sufficiently constrained for non-copyable iterators","Prague","|Complete|","15.0","|ranges|"
 "`3387 <https://wg21.link/LWG3387>`__","|sect|\ [range.reverse.view] ``reverse_view<V>``\  unintentionally requires ``range<const V>``\ ","Prague","|Complete|","15.0","|ranges|"
diff --git a/libcxx/docs/Status/Cxx20Papers.csv b/libcxx/docs/Status/Cxx20Papers.csv
index db64914..77078b1 100644
--- a/libcxx/docs/Status/Cxx20Papers.csv
+++ b/libcxx/docs/Status/Cxx20Papers.csv
@@ -179,7 +179,7 @@
 "`P1970R2 <https://wg21.link/P1970R2>`__","LWG","Consistency for size() functions: Add ranges::ssize","Prague","|Complete|","15.0","|ranges|"
 "`P1973R1 <https://wg21.link/P1973R1>`__","LWG","Rename ""_default_init"" Functions, Rev1","Prague","|Complete|","16.0"
 "`P1976R2 <https://wg21.link/P1976R2>`__","LWG","Fixed-size span construction from dynamic range","Prague","|Complete|","11.0","|ranges|"
-"`P1981R0 <https://wg21.link/P1981R0>`__","LWG","Rename leap to leap_second","Prague","* *",""
+"`P1981R0 <https://wg21.link/P1981R0>`__","LWG","Rename leap to leap_second","Prague","|Complete|","19.0","|chrono|"
 "`P1982R0 <https://wg21.link/P1982R0>`__","LWG","Rename link to time_zone_link","Prague","|Complete|","19.0","|chrono|"
 "`P1983R0 <https://wg21.link/P1983R0>`__","LWG","Wording for GB301, US296, US292, US291, and US283","Prague","|Complete|","15.0","|ranges|"
 "`P1994R1 <https://wg21.link/P1994R1>`__","LWG","elements_view needs its own sentinel","Prague","|Complete|","16.0","|ranges|"
diff --git a/libcxx/docs/Status/SpaceshipProjects.csv b/libcxx/docs/Status/SpaceshipProjects.csv
index c822107..3d14f48 100644
--- a/libcxx/docs/Status/SpaceshipProjects.csv
+++ b/libcxx/docs/Status/SpaceshipProjects.csv
@@ -173,7 +173,7 @@ Section,Description,Dependencies,Assignee,Complete
 | `year_month_weekday_last <https://reviews.llvm.org/D152699>`_",None,Hristo Hristov,|Complete|
 `[time.zone.nonmembers] <https://wg21.link/time.zone.nonmembers>`_,"`chrono::time_zone`",A ``<chrono>`` implementation,Mark de Wever,|Complete|
 `[time.zone.zonedtime.nonmembers] <https://wg21.link/time.zone.zonedtime.nonmembers>`_,"`chrono::zoned_time`",A ``<chrono>`` implementation,Mark de Wever,|In Progress|
-`[time.zone.leap.nonmembers] <https://wg21.link/time.zone.leap.nonmembers>`_,"`chrono::time_leap_seconds`",A ``<chrono>`` implementation,Mark de Wever,|In Progress|
+`[time.zone.leap.nonmembers] <https://wg21.link/time.zone.leap.nonmembers>`_,"`chrono::time_leap_seconds`",A ``<chrono>`` implementation,Mark de Wever,|Complete|
 `[time.zone.link.nonmembers] <https://wg21.link/time.zone.link.nonmembers>`_,"`chrono::time_zone_link`",A ``<chrono>`` implementation,Mark de Wever,|Complete|
 - `5.13 Clause 28: Localization library <https://wg21.link/p1614r2#clause-28-localization-library>`_,,,,
 "| `[locale] <https://wg21.link/locale>`_
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index b935e45..db39803 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -282,6 +282,7 @@ set(files
   __chrono/formatter.h
   __chrono/hh_mm_ss.h
   __chrono/high_resolution_clock.h
+  __chrono/leap_second.h
   __chrono/literals.h
   __chrono/month.h
   __chrono/month_weekday.h
diff --git a/libcxx/include/__chrono/leap_second.h b/libcxx/include/__chrono/leap_second.h
new file mode 100644
index 0000000..4e67cc2
--- /dev/null
+++ b/libcxx/include/__chrono/leap_second.h
@@ -0,0 +1,126 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// For information see https://libcxx.llvm.org/DesignDocs/TimeZone.html
+
+#ifndef _LIBCPP___CHRONO_LEAP_SECOND_H
+#define _LIBCPP___CHRONO_LEAP_SECOND_H
+
+#include <version>
+// Enable the contents of the header only when libc++ was built with experimental features enabled.
+#if !defined(_LIBCPP_HAS_NO_INCOMPLETE_TZDB)
+
+#  include <__chrono/duration.h>
+#  include <__chrono/system_clock.h>
+#  include <__chrono/time_point.h>
+#  include <__compare/ordering.h>
+#  include <__compare/three_way_comparable.h>
+#  include <__config>
+
+#  if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
+#    pragma GCC system_header
+#  endif
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+#  if _LIBCPP_STD_VER >= 20
+
+namespace chrono {
+
+class leap_second {
+public:
+  struct __constructor_tag;
+  [[nodiscard]]
+  _LIBCPP_HIDE_FROM_ABI explicit constexpr leap_second(__constructor_tag&&, sys_seconds __date, seconds __value)
+      : __date_(__date), __value_(__value) {}
+
+  _LIBCPP_HIDE_FROM_ABI leap_second(const leap_second&)            = default;
+  _LIBCPP_HIDE_FROM_ABI leap_second& operator=(const leap_second&) = default;
+
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr sys_seconds date() const noexcept { return __date_; }
+
+  _LIBCPP_NODISCARD_EXT _LIBCPP_HIDE_FROM_ABI constexpr seconds value() const noexcept { return __value_; }
+
+private:
+  sys_seconds __date_;
+  seconds __value_;
+};
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr bool operator==(const leap_second& __x, const leap_second& __y) {
+  return __x.date() == __y.date();
+}
+
+_LIBCPP_HIDE_FROM_ABI inline constexpr strong_ordering operator<=>(const leap_second& __x, const leap_second& __y) {
+  return __x.date() <=> __y.date();
+}
+
+template <class _Duration>
+_LIBCPP_HIDE_FROM_ABI constexpr bool operator==(const leap_second& __x, const sys_time<_Duration>& __y) {
+  return __x.date() == __y;
+}
+
+template <class _Duration>
+_LIBCPP_HIDE_FROM_ABI constexpr bool operator<(const leap_second& __x, const sys_time<_Duration>& __y) {
+  return __x.date() < __y;
+}
+
+template <class _Duration>
+_LIBCPP_HIDE_FROM_ABI constexpr bool operator<(const sys_time<_Duration>& __x, const leap_second& __y) {
+  return __x < __y.date();
+}
+
+template <class _Duration>
+_LIBCPP_HIDE_FROM_ABI constexpr bool operator>(const leap_second& __x, const sys_time<_Duration>& __y) {
+  return __y < __x;
+}
+
+template <class _Duration>
+_LIBCPP_HIDE_FROM_ABI constexpr bool operator>(const sys_time<_Duration>& __x, const leap_second& __y) {
+  return __y < __x;
+}
+
+template <class _Duration>
+_LIBCPP_HIDE_FROM_ABI constexpr bool operator<=(const leap_second& __x, const sys_time<_Duration>& __y) {
+  return !(__y < __x);
+}
+
+template <class _Duration>
+_LIBCPP_HIDE_FROM_ABI constexpr bool operator<=(const sys_time<_Duration>& __x, const leap_second& __y) {
+  return !(__y < __x);
+}
+
+template <class _Duration>
+_LIBCPP_HIDE_FROM_ABI constexpr bool operator>=(const leap_second& __x, const sys_time<_Duration>& __y) {
+  return !(__x < __y);
+}
+
+template <class _Duration>
+_LIBCPP_HIDE_FROM_ABI constexpr bool operator>=(const sys_time<_Duration>& __x, const leap_second& __y) {
+  return !(__x < __y);
+}
+
+#    ifndef _LIBCPP_COMPILER_GCC
+// This requirement cause a compilation loop in GCC-13 and running out of memory.
+// TODO TZDB Test whether GCC-14 fixes this.
+template <class _Duration>
+  requires three_way_comparable_with<sys_seconds, sys_time<_Duration>>
+_LIBCPP_HIDE_FROM_ABI constexpr auto operator<=>(const leap_second& __x, const sys_time<_Duration>& __y) {
+  return __x.date() <=> __y;
+}
+#    endif
+
+} // namespace chrono
+
+#  endif //_LIBCPP_STD_VER >= 20
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // !defined(_LIBCPP_HAS_NO_INCOMPLETE_TZDB)
+
+#endif // _LIBCPP___CHRONO_LEAP_SECOND_H
diff --git a/libcxx/include/__chrono/tzdb.h b/libcxx/include/__chrono/tzdb.h
index 582172e..45c20f2 100644
--- a/libcxx/include/__chrono/tzdb.h
+++ b/libcxx/include/__chrono/tzdb.h
@@ -16,6 +16,7 @@
 // Enable the contents of the header only when libc++ was built with experimental features enabled.
 #if !defined(_LIBCPP_HAS_NO_INCOMPLETE_TZDB)
 
+#  include <__chrono/leap_second.h>
 #  include <__chrono/time_zone.h>
 #  include <__chrono/time_zone_link.h>
 #  include <__config>
@@ -40,6 +41,8 @@ struct tzdb {
   string version;
   vector<time_zone> zones;
   vector<time_zone_link> links;
+
+  vector<leap_second> leap_seconds;
 };
 
 } // namespace chrono
diff --git a/libcxx/include/chrono b/libcxx/include/chrono
index 5bab3f8..4dd4313 100644
--- a/libcxx/include/chrono
+++ b/libcxx/include/chrono
@@ -688,6 +688,7 @@ struct tzdb {
   string                 version;
   vector<time_zone>      zones;
   vector<time_zone_link> links;
+  vector<leap_second>    leap_seconds;
 };
 
 class tzdb_list {                                                                // C++20
@@ -731,6 +732,43 @@ class time_zone {
 bool operator==(const time_zone& x, const time_zone& y) noexcept;                // C++20
 strong_ordering operator<=>(const time_zone& x, const time_zone& y) noexcept;    // C++20
 
+// [time.zone.leap], leap second support
+class leap_second {                                                              // C++20
+public:
+  leap_second(const leap_second&)            = default;
+  leap_second& operator=(const leap_second&) = default;
+
+  // unspecified additional constructors
+
+  constexpr sys_seconds date() const noexcept;
+  constexpr seconds value() const noexcept;
+};
+
+constexpr bool operator==(const leap_second& x, const leap_second& y);           // C++20
+constexpr strong_ordering operator<=>(const leap_second& x, const leap_second& y);
+
+template<class Duration>                                                         // C++20
+  constexpr bool operator==(const leap_second& x, const sys_time<Duration>& y);
+template<class Duration>                                                         // C++20
+  constexpr bool operator< (const leap_second& x, const sys_time<Duration>& y);
+template<class Duration>                                                         // C++20
+  constexpr bool operator< (const sys_time<Duration>& x, const leap_second& y);
+template<class Duration>                                                         // C++20
+  constexpr bool operator> (const leap_second& x, const sys_time<Duration>& y);
+template<class Duration>                                                         // C++20
+  constexpr bool operator> (const sys_time<Duration>& x, const leap_second& y);
+template<class Duration>                                                         // C++20
+  constexpr bool operator<=(const leap_second& x, const sys_time<Duration>& y);
+template<class Duration>                                                         // C++20
+  constexpr bool operator<=(const sys_time<Duration>& x, const leap_second& y);
+template<class Duration>                                                         // C++20
+  constexpr bool operator>=(const leap_second& x, const sys_time<Duration>& y);
+template<class Duration>                                                         // C++20
+  constexpr bool operator>=(const sys_time<Duration>& x, const leap_second& y);
+template<class Duration>                                                         // C++20
+  requires three_way_comparable_with<sys_seconds, sys_time<Duration>>
+  constexpr auto operator<=>(const leap_second& x, const sys_time<Duration>& y);
+
 // [time.zone.link], class time_zone_link
 class time_zone_link {                                                           // C++20
 public:
@@ -862,6 +900,7 @@ constexpr chrono::year                                  operator ""y(unsigned lo
 
 #if !defined(_LIBCPP_HAS_NO_TIME_ZONE_DATABASE) && !defined(_LIBCPP_HAS_NO_FILESYSTEM) &&                              \
     !defined(_LIBCPP_HAS_NO_LOCALIZATION)
+#  include <__chrono/leap_second.h>
 #  include <__chrono/time_zone.h>
 #  include <__chrono/time_zone_link.h>
 #  include <__chrono/tzdb.h>
diff --git a/libcxx/include/libcxx.imp b/libcxx/include/libcxx.imp
index ea0ba8c..2cb1fa5 100644
--- a/libcxx/include/libcxx.imp
+++ b/libcxx/include/libcxx.imp
@@ -279,6 +279,7 @@
   { include: [ "<__chrono/formatter.h>", "private", "<chrono>", "public" ] },
   { include: [ "<__chrono/hh_mm_ss.h>", "private", "<chrono>", "public" ] },
   { include: [ "<__chrono/high_resolution_clock.h>", "private", "<chrono>", "public" ] },
+  { include: [ "<__chrono/leap_second.h>", "private", "<chrono>", "public" ] },
   { include: [ "<__chrono/literals.h>", "private", "<chrono>", "public" ] },
   { include: [ "<__chrono/month.h>", "private", "<chrono>", "public" ] },
   { include: [ "<__chrono/month_weekday.h>", "private", "<chrono>", "public" ] },
diff --git a/libcxx/include/module.modulemap b/libcxx/include/module.modulemap
index 22c3803..6d4dcc2 100644
--- a/libcxx/include/module.modulemap
+++ b/libcxx/include/module.modulemap
@@ -1145,6 +1145,7 @@ module std_private_chrono_high_resolution_clock  [system] {
   export std_private_chrono_steady_clock
   export std_private_chrono_system_clock
 }
+module std_private_chrono_leap_second            [system] { header "__chrono/leap_second.h" }
 module std_private_chrono_literals               [system] { header "__chrono/literals.h" }
 module std_private_chrono_month                  [system] { header "__chrono/month.h" }
 module std_private_chrono_month_weekday          [system] { header "__chrono/month_weekday.h" }
diff --git a/libcxx/modules/std/chrono.inc b/libcxx/modules/std/chrono.inc
index 109023a..2c0bd3f 100644
--- a/libcxx/modules/std/chrono.inc
+++ b/libcxx/modules/std/chrono.inc
@@ -208,10 +208,7 @@ export namespace std {
     using std::chrono::reload_tzdb;
     using std::chrono::remote_version;
 
-#  endif //  !defined(_LIBCPP_HAS_NO_TIME_ZONE_DATABASE) && !defined(_LIBCPP_HAS_NO_FILESYSTEM) &&
-         //    !defined(_LIBCPP_HAS_NO_LOCALIZATION)
-
-#  if 0
+#    if 0
     // [time.zone.exception], exception classes
     using std::chrono::ambiguous_local_time;
     using std::chrono::nonexistent_local_time;
@@ -221,11 +218,11 @@ export namespace std {
 
     // [time.zone.timezone], class time_zone
     using std::chrono::choose;
-#  endif
-#  ifdef _LIBCPP_ENABLE_EXPERIMENTAL
+#    endif // if 0
+
     using std::chrono::time_zone;
-#  endif
-#  if 0
+
+#    if 0
 
     // [time.zone.zonedtraits], class template zoned_traits
     using std::chrono::zoned_traits;
@@ -234,22 +231,23 @@ export namespace std {
     using std::chrono::zoned_time;
 
     using std::chrono::zoned_seconds;
+#    endif // if 0
 
     // [time.zone.leap], leap second support
     using std::chrono::leap_second;
-#  endif
 
-#  ifdef _LIBCPP_ENABLE_EXPERIMENTAL
     // [time.zone.link], class time_zone_link
     using std::chrono::time_zone_link;
-#  endif
 
-#  if 0
+#    if 0
     // [time.format], formatting
     using std::chrono::local_time_format;
-#  endif
-#endif // _LIBCPP_ENABLE_EXPERIMENTAL
-  }    // namespace chrono
+#    endif
+#  endif // _LIBCPP_ENABLE_EXPERIMENTAL
+#endif   //  !defined(_LIBCPP_HAS_NO_TIME_ZONE_DATABASE) && !defined(_LIBCPP_HAS_NO_FILESYSTEM) &&
+         //    !defined(_LIBCPP_HAS_NO_LOCALIZATION)
+
+  } // namespace chrono
 
 #ifndef _LIBCPP_HAS_NO_LOCALIZATION
   using std::formatter;
diff --git a/libcxx/src/CMakeLists.txt b/libcxx/src/CMakeLists.txt
index 1110a79..16ccb80 100644
--- a/libcxx/src/CMakeLists.txt
+++ b/libcxx/src/CMakeLists.txt
@@ -334,6 +334,7 @@ endif()
 
 if (LIBCXX_ENABLE_LOCALIZATION AND LIBCXX_ENABLE_FILESYSTEM AND LIBCXX_ENABLE_TIME_ZONE_DATABASE)
   list(APPEND LIBCXX_EXPERIMENTAL_SOURCES
+    include/tzdb/leap_second_private.h
     include/tzdb/time_zone_link_private.h
     include/tzdb/time_zone_private.h
     include/tzdb/types_private.h
diff --git a/libcxx/src/include/tzdb/leap_second_private.h b/libcxx/src/include/tzdb/leap_second_private.h
new file mode 100644
index 0000000..7a811ab
--- /dev/null
+++ b/libcxx/src/include/tzdb/leap_second_private.h
@@ -0,0 +1,27 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// For information see https://libcxx.llvm.org/DesignDocs/TimeZone.html
+
+#ifndef _LIBCPP_SRC_INCLUDE_TZDB_LEAP_SECOND_PRIVATE_H
+#define _LIBCPP_SRC_INCLUDE_TZDB_LEAP_SECOND_PRIVATE_H
+
+#include <chrono>
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+namespace chrono {
+
+struct leap_second::__constructor_tag {};
+
+} // namespace chrono
+
+_LIBCPP_END_NAMESPACE_STD
+
+#endif // _LIBCPP_SRC_INCLUDE_TZDB_LEAP_SECOND_PRIVATE_H
diff --git a/libcxx/src/tzdb.cpp b/libcxx/src/tzdb.cpp
index 0307f754..7ba5ceb 100644
--- a/libcxx/src/tzdb.cpp
+++ b/libcxx/src/tzdb.cpp
@@ -15,6 +15,7 @@
 #include <stdexcept>
 #include <string>
 
+#include "include/tzdb/leap_second_private.h"
 #include "include/tzdb/time_zone_link_private.h"
 #include "include/tzdb/time_zone_private.h"
 #include "include/tzdb/types_private.h"
@@ -622,6 +623,36 @@ static void __parse_tzdata(tzdb& __db, __tz::__rules_storage_type& __rules, istr
   }
 }
 
+static void __parse_leap_seconds(vector<leap_second>& __leap_seconds, istream&& __input) {
+  // The file stores dates since 1 January 1900, 00:00:00, we want
+  // seconds since 1 January 1970.
+  constexpr auto __offset = sys_days{1970y / January / 1} - sys_days{1900y / January / 1};
+
+  while (true) {
+    switch (__input.peek()) {
+    case istream::traits_type::eof():
+      return;
+
+    case ' ':
+    case '\t':
+    case '\n':
+      __input.get();
+      continue;
+
+    case '#':
+      chrono::__skip_line(__input);
+      continue;
+    }
+
+    sys_seconds __date = sys_seconds{seconds{chrono::__parse_integral(__input, false)}} - __offset;
+    chrono::__skip_mandatory_whitespace(__input);
+    seconds __value{chrono::__parse_integral(__input, false)};
+    chrono::__skip_line(__input);
+
+    __leap_seconds.emplace_back(leap_second::__constructor_tag{}, __date, __value);
+  }
+}
+
 void __init_tzdb(tzdb& __tzdb, __tz::__rules_storage_type& __rules) {
   filesystem::path __root = chrono::__libcpp_tzdb_directory();
   ifstream __tzdata{__root / "tzdata.zi"};
@@ -631,6 +662,17 @@ void __init_tzdb(tzdb& __tzdb, __tz::__rules_storage_type& __rules) {
   std::ranges::sort(__tzdb.zones);
   std::ranges::sort(__tzdb.links);
   std::ranges::sort(__rules, {}, [](const auto& p) { return p.first; });
+
+  // There are two files with the leap second information
+  // - leapseconds as specified by zic
+  // - leap-seconds.list the source data
+  // The latter is much easier to parse, it seems Howard shares that
+  // opinion.
+  chrono::__parse_leap_seconds(__tzdb.leap_seconds, ifstream{__root / "leap-seconds.list"});
+  // The Standard requires the leap seconds to be sorted. The file
+  // leap-seconds.list usually provides them in sorted order, but that is not
+  // guaranteed so we ensure it here.
+  std::ranges::sort(__tzdb.leap_seconds);
 }
 
 //===----------------------------------------------------------------------===//
diff --git a/libcxx/test/libcxx/diagnostics/chrono.nodiscard_extensions.compile.pass.cpp b/libcxx/test/libcxx/diagnostics/chrono.nodiscard_extensions.compile.pass.cpp
index 8019824..c868832 100644
--- a/libcxx/test/libcxx/diagnostics/chrono.nodiscard_extensions.compile.pass.cpp
+++ b/libcxx/test/libcxx/diagnostics/chrono.nodiscard_extensions.compile.pass.cpp
@@ -26,6 +26,7 @@
 // These types have "private" constructors.
 extern std::chrono::time_zone tz;
 extern std::chrono::time_zone_link link;
+extern std::chrono::leap_second leap;
 
 void test() {
   std::chrono::tzdb_list& list = std::chrono::get_tzdb_list();
@@ -51,4 +52,9 @@ void test() {
     operator==(link, link);
     operator<=>(link, link);
   }
+
+  {
+    leap.date();
+    leap.value();
+  }
 }
diff --git a/libcxx/test/libcxx/diagnostics/chrono.nodiscard_extensions.verify.cpp b/libcxx/test/libcxx/diagnostics/chrono.nodiscard_extensions.verify.cpp
index e9b2755..4d26b46 100644
--- a/libcxx/test/libcxx/diagnostics/chrono.nodiscard_extensions.verify.cpp
+++ b/libcxx/test/libcxx/diagnostics/chrono.nodiscard_extensions.verify.cpp
@@ -23,6 +23,7 @@
 // These types have "private" constructors.
 extern std::chrono::time_zone tz;
 extern std::chrono::time_zone_link link;
+extern std::chrono::leap_second leap;
 
 void test() {
   std::chrono::tzdb_list& list = std::chrono::get_tzdb_list();
@@ -51,4 +52,9 @@ void test() {
     // expected-warning@+1 {{ignoring return value of function declared with 'nodiscard' attribute}}
     operator<=>(link, link);
   }
+
+  {
+    leap.date();  // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+    leap.value(); // expected-warning {{ignoring return value of function declared with 'nodiscard' attribute}}
+  }
 }
diff --git a/libcxx/test/libcxx/time/time.zone/time.zone.db/leap_seconds.pass.cpp b/libcxx/test/libcxx/time/time.zone/time.zone.db/leap_seconds.pass.cpp
new file mode 100644
index 0000000..282bddc
--- /dev/null
+++ b/libcxx/test/libcxx/time/time.zone/time.zone.db/leap_seconds.pass.cpp
@@ -0,0 +1,119 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-filesystem, no-localization, no-tzdb
+
+// XFAIL: libcpp-has-no-incomplete-tzdb
+// XFAIL: availability-tzdb-missing
+
+// <chrono>
+
+// Tests the IANA database leap seconds parsing and operations.
+// This is not part of the public tzdb interface.
+
+#include <cassert>
+#include <chrono>
+#include <fstream>
+#include <string>
+#include <string_view>
+
+#include "assert_macros.h"
+#include "concat_macros.h"
+#include "filesystem_test_helper.h"
+#include "test_tzdb.h"
+
+scoped_test_env env;
+[[maybe_unused]] const std::filesystem::path dir = env.create_dir("zoneinfo");
+const std::filesystem::path tzdata               = env.create_file("zoneinfo/tzdata.zi");
+const std::filesystem::path leap_seconds         = env.create_file("zoneinfo/leap-seconds.list");
+
+std::string_view std::chrono::__libcpp_tzdb_directory() {
+  static std::string result = dir.string();
+  return result;
+}
+
+void write(std::string_view input) {
+  static int version = 0;
+
+  std::ofstream f{tzdata};
+  f << "# version " << version++ << '\n';
+  std::ofstream{leap_seconds}.write(input.data(), input.size());
+}
+
+static const std::chrono::tzdb& parse(std::string_view input) {
+  write(input);
+  return std::chrono::reload_tzdb();
+}
+
+static void test_exception(std::string_view input, [[maybe_unused]] std::string_view what) {
+  write(input);
+
+  TEST_VALIDATE_EXCEPTION(
+      std::runtime_error,
+      [&]([[maybe_unused]] const std::runtime_error& e) {
+        TEST_LIBCPP_REQUIRE(
+            e.what() == what,
+            TEST_WRITE_CONCATENATED("\nExpected exception ", what, "\nActual exception   ", e.what(), '\n'));
+      },
+      TEST_IGNORE_NODISCARD std::chrono::reload_tzdb());
+}
+
+static void test_invalid() {
+  test_exception("0", "corrupt tzdb: expected a non-zero digit");
+
+  test_exception("1", "corrupt tzdb: expected whitespace");
+
+  test_exception("1 ", "corrupt tzdb: expected a non-zero digit");
+
+  test_exception("5764607523034234880 2", "corrupt tzdb: integral too large");
+}
+
+static void test_leap_seconds() {
+  using namespace std::chrono;
+
+  // Test whether loading also sorts the entries in the proper order.
+  const tzdb& result = parse(
+      R"(
+2303683200  12  # 1 Jan 1973
+2287785600  11  # 1 Jul 1972
+2272060800  10  # 1 Jan 1972
+86400        1  # 2 Jan 1900 Dummy entry to test before 1970
+
+# largest accepted value by the parser
+5764607523034234879 2
+)");
+
+  assert(result.leap_seconds.size() == 5);
+
+  assert(result.leap_seconds[0].date() == sys_seconds{sys_days{1900y / January / 2}});
+  assert(result.leap_seconds[0].value() == 1s);
+
+  assert(result.leap_seconds[1].date() == sys_seconds{sys_days{1972y / January / 1}});
+  assert(result.leap_seconds[1].value() == 10s);
+
+  assert(result.leap_seconds[2].date() == sys_seconds{sys_days{1972y / July / 1}});
+  assert(result.leap_seconds[2].value() == 11s);
+
+  assert(result.leap_seconds[3].date() == sys_seconds{sys_days{1973y / January / 1}});
+  assert(result.leap_seconds[3].value() == 12s);
+
+  assert(result.leap_seconds[4].date() ==
+         sys_seconds{5764607523034234879s
+                     // The database uses 1900-01-01 as epoch.
+                     - std::chrono::duration_cast<std::chrono::seconds>(
+                           sys_days{1970y / January / 1} - sys_days{1900y / January / 1})});
+  assert(result.leap_seconds[4].value() == 2s);
+}
+
+int main(int, const char**) {
+  test_invalid();
+  test_leap_seconds();
+
+  return 0;
+}
diff --git a/libcxx/test/std/time/time.zone/time.zone.db/leap_seconds.pass.cpp b/libcxx/test/std/time/time.zone/time.zone.db/leap_seconds.pass.cpp
new file mode 100644
index 0000000..4fcdf6f
--- /dev/null
+++ b/libcxx/test/std/time/time.zone/time.zone.db/leap_seconds.pass.cpp
@@ -0,0 +1,75 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-filesystem, no-localization, no-tzdb
+
+// XFAIL: libcpp-has-no-incomplete-tzdb
+// XFAIL: availability-tzdb-missing
+
+// <chrono>
+
+// Tests the loaded leap seconds match
+// https://eel.is/c++draft/time.zone.leap.overview#2
+//
+// At the moment of writing that list is the actual list.
+// If in the future more leap seconds are added, the returned list may have more
+
+#include <algorithm>
+#include <array>
+#include <cassert>
+#include <chrono>
+#include <ranges>
+
+using namespace std::literals::chrono_literals;
+
+// The list of leap seconds matching
+// https://eel.is/c++draft/time.zone.leap.overview#2
+// At the moment of writing that list is the actual list in the IANA database.
+// If in the future more leap seconds can be added.
+static const std::array leap_seconds = {
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1972y / std::chrono::January / 1}}, 10s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1972y / std::chrono::July / 1}}, 11s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1973y / std::chrono::January / 1}}, 12s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1974y / std::chrono::January / 1}}, 13s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1975y / std::chrono::January / 1}}, 14s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1976y / std::chrono::January / 1}}, 15s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1977y / std::chrono::January / 1}}, 16s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1978y / std::chrono::January / 1}}, 17s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1979y / std::chrono::January / 1}}, 18s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1980y / std::chrono::January / 1}}, 19s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1981y / std::chrono::July / 1}}, 20s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1982y / std::chrono::July / 1}}, 21s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1983y / std::chrono::July / 1}}, 22s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1985y / std::chrono::July / 1}}, 23s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1988y / std::chrono::January / 1}}, 24s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1990y / std::chrono::January / 1}}, 25s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1991y / std::chrono::January / 1}}, 26s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1992y / std::chrono::July / 1}}, 27s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1993y / std::chrono::July / 1}}, 28s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1994y / std::chrono::July / 1}}, 29s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1996y / std::chrono::January / 1}}, 30s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1997y / std::chrono::July / 1}}, 31s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{1999y / std::chrono::January / 1}}, 32s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{2006y / std::chrono::January / 1}}, 33s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{2009y / std::chrono::January / 1}}, 34s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{2012y / std::chrono::July / 1}}, 35s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{2015y / std::chrono::July / 1}}, 36s),
+    std::make_pair(std::chrono::sys_seconds{std::chrono::sys_days{2017y / std::chrono::January / 1}}, 37s)};
+
+int main(int, const char**) {
+  const std::chrono::tzdb& tzdb = std::chrono::get_tzdb();
+
+  assert(tzdb.leap_seconds.size() >= leap_seconds.size());
+  assert((std::ranges::equal(
+      leap_seconds,
+      tzdb.leap_seconds | std::ranges::views::take(leap_seconds.size()),
+      [](const auto& lhs, const auto& rhs) { return lhs.first == rhs.date() && lhs.second == rhs.value(); })));
+
+  return 0;
+}
diff --git a/libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.access/get_tzdb.pass.cpp b/libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.access/get_tzdb.pass.cpp
index b6204c6..470a722 100644
--- a/libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.access/get_tzdb.pass.cpp
+++ b/libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.access/get_tzdb.pass.cpp
@@ -35,5 +35,8 @@ int main(int, const char**) {
   assert(std::ranges::is_sorted(db.links));
   assert(std::ranges::adjacent_find(db.links) == db.links.end()); // is unique?
 
+  assert(!db.leap_seconds.empty());
+  assert(std::ranges::is_sorted(db.leap_seconds));
+
   return 0;
 }
diff --git a/libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.tzdb/tzdb.members.pass.cpp b/libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.tzdb/tzdb.members.pass.cpp
index 51c6d36..af95274 100644
--- a/libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.tzdb/tzdb.members.pass.cpp
+++ b/libcxx/test/std/time/time.zone/time.zone.db/time.zone.db.tzdb/tzdb.members.pass.cpp
@@ -37,11 +37,9 @@ int main(int, const char**) {
   tzdb.version = "version";
   assert(tzdb.version == "version");
 
-  [[maybe_unused]] std::vector<std::chrono::time_zone>& zones = tzdb.zones;
-
+  [[maybe_unused]] std::vector<std::chrono::time_zone>& zones          = tzdb.zones;
   [[maybe_unused]] std::vector<std::chrono::time_zone_link>& links = tzdb.links;
-
-  // TODO TZDB add the leap data member
+  [[maybe_unused]] std::vector<std::chrono::leap_second>& leap_seconds = tzdb.leap_seconds;
 
   return 0;
 }
diff --git a/libcxx/test/std/time/time.zone/time.zone.leap/assign.copy.pass.cpp b/libcxx/test/std/time/time.zone/time.zone.leap/assign.copy.pass.cpp
new file mode 100644
index 0000000..4d91e73
--- /dev/null
+++ b/libcxx/test/std/time/time.zone/time.zone.leap/assign.copy.pass.cpp
@@ -0,0 +1,71 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-filesystem, no-localization, no-tzdb
+
+// XFAIL: libcpp-has-no-incomplete-tzdb
+// XFAIL: availability-tzdb-missing
+
+// <chrono>
+
+// class leap_second
+// {
+//   leap_second& operator=(const leap_second&) = default;
+//
+//   ...
+// };
+
+#include <chrono>
+#include <concepts>
+#include <memory>
+#include <type_traits>
+#include <cassert>
+
+// Add the include path required by test_chrono_leap_second.h when using libc++.
+// ADDITIONAL_COMPILE_FLAGS(stdlib=libc++): -I %{libcxx-dir}/src/include
+#include "test_chrono_leap_second.h"
+
+constexpr bool test() {
+  std::chrono::leap_second a =
+      test_leap_second_create(std::chrono::sys_seconds{std::chrono::seconds{0}}, std::chrono::seconds{1});
+  std::chrono::leap_second b =
+      test_leap_second_create(std::chrono::sys_seconds{std::chrono::seconds{10}}, std::chrono::seconds{15});
+
+  //  operator== only compares the date member.
+  assert(a.date() != b.date());
+  assert(a.value() != b.value());
+
+  {
+    std::same_as<std::chrono::leap_second&> decltype(auto) result(b = a);
+    assert(std::addressof(result) == std::addressof(b));
+
+    assert(a.date() == b.date());
+    assert(a.value() == b.value());
+  }
+
+  {
+    // Tests an rvalue uses the copy assignment.
+    std::same_as<std::chrono::leap_second&> decltype(auto) result(b = std::move(a));
+    assert(std::addressof(result) == std::addressof(b));
+
+    assert(a.date() == b.date());
+    assert(a.value() == b.value());
+  }
+
+  return true;
+}
+
+int main(int, const char**) {
+  static_assert(std::is_copy_assignable_v<std::chrono::leap_second>);
+
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/time/time.zone/time.zone.leap/cons.copy.pass.cpp b/libcxx/test/std/time/time.zone/time.zone.leap/cons.copy.pass.cpp
new file mode 100644
index 0000000..e2419b7
--- /dev/null
+++ b/libcxx/test/std/time/time.zone/time.zone.leap/cons.copy.pass.cpp
@@ -0,0 +1,69 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-filesystem, no-localization, no-tzdb
+
+// XFAIL: libcpp-has-no-incomplete-tzdb
+// XFAIL: availability-tzdb-missing
+
+// <chrono>
+
+// class leap_second
+// {
+//   leap_second(const leap_second&)            = default;
+//
+//   ...
+// };
+
+#include <chrono>
+#include <concepts>
+#include <cassert>
+
+// Add the include path required by test_chrono_leap_second.h when using libc++.
+// ADDITIONAL_COMPILE_FLAGS(stdlib=libc++): -I %{libcxx-dir}/src/include
+#include "test_chrono_leap_second.h"
+
+constexpr bool test() {
+  std::chrono::leap_second a =
+      test_leap_second_create(std::chrono::sys_seconds{std::chrono::seconds{0}}, std::chrono::seconds{1});
+
+  {
+    std::chrono::leap_second b = a;
+
+    //  operator== only compares the date member.
+    assert(a.date() == b.date());
+    assert(a.value() == b.value());
+  }
+
+#ifdef _LIBCPP_VERSION
+  {
+    // Tests an rvalue uses the copy constructor.
+    // Since implementations are allowed to add additional constructors this is
+    // a libc++ specific test.
+    std::chrono::leap_second b = std::move(a);
+
+    //  operator== only compares the date member.
+    assert(a.date() == b.date());
+    assert(a.value() == b.value());
+  }
+  // libc++ does not provide a default constructor.
+  static_assert(!std::is_default_constructible_v<std::chrono::leap_second>);
+#endif // _LIBCPP_VERSION
+
+  return true;
+}
+
+int main(int, const char**) {
+  static_assert(std::copy_constructible<std::chrono::leap_second>);
+
+  test();
+  static_assert(test());
+
+  return 0;
+}
diff --git a/libcxx/test/std/time/time.zone/time.zone.leap/members/date.pass.cpp b/libcxx/test/std/time/time.zone/time.zone.leap/members/date.pass.cpp
new file mode 100644
index 0000000..23f95ec
--- /dev/null
+++ b/libcxx/test/std/time/time.zone/time.zone.leap/members/date.pass.cpp
@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-filesystem, no-localization, no-tzdb
+
+// XFAIL: libcpp-has-no-incomplete-tzdb
+// XFAIL: availability-tzdb-missing
+
+// <chrono>
+
+// class leap_second;
+
+// constexpr sys_seconds date() const noexcept;
+
+#include <cassert>
+#include <chrono>
+
+#include "test_macros.h"
+
+// Add the include path required by test_chrono_leap_second.h when using libc++.
+// ADDITIONAL_COMPILE_FLAGS(stdlib=libc++): -I %{libcxx-dir}/src/include
+#include "test_chrono_leap_second.h"
+
+constexpr void test(const std::chrono::leap_second leap_second, std::chrono::sys_seconds expected) {
+  std::same_as<std::chrono::sys_seconds> auto date = leap_second.date();
+  assert(date == expected);
+  static_assert(noexcept(leap_second.date()));
+}
+
+constexpr bool test() {
+  test(test_leap_second_create(std::chrono::sys_seconds{std::chrono::seconds{0}}, std::chrono::seconds{1}),
+       std::chrono::sys_seconds{std::chrono::seconds{0}});
+
+  return true;
+}
+
+int main(int, const char**) {
+  test();
+  static_assert(test());
+
+  // test with the real tzdb
+  const std::chrono::tzdb& tzdb = std::chrono::get_tzdb();
+  assert(!tzdb.leap_seconds.empty());
+  test(tzdb.leap_seconds[0], tzdb.leap_seconds[0].date());
+
+  return 0;
+}
diff --git a/libcxx/test/std/time/time.zone/time.zone.leap/members/value.pass.cpp b/libcxx/test/std/time/time.zone/time.zone.leap/members/value.pass.cpp
new file mode 100644
index 0000000..844c74d
--- /dev/null
+++ b/libcxx/test/std/time/time.zone/time.zone.leap/members/value.pass.cpp
@@ -0,0 +1,53 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-filesystem, no-localization, no-tzdb
+
+// XFAIL: libcpp-has-no-incomplete-tzdb
+// XFAIL: availability-tzdb-missing
+
+// <chrono>
+
+// class leap_second;
+
+// constexpr seconds value() const noexcept;
+
+#include <cassert>
+#include <chrono>
+
+#include "test_macros.h"
+
+// Add the include path required by test_chrono_leap_second.h when using libc++.
+// ADDITIONAL_COMPILE_FLAGS(stdlib=libc++): -I %{libcxx-dir}/src/include
+#include "test_chrono_leap_second.h"
+
+constexpr void test(const std::chrono::leap_second leap_second, std::chrono::seconds expected) {
+  std::same_as<std::chrono::seconds> auto value = leap_second.value();
+  assert(value == expected);
+  static_assert(noexcept(leap_second.value()));
+}
+
+constexpr bool test() {
+  test(test_leap_second_create(std::chrono::sys_seconds{std::chrono::seconds{0}}, std::chrono::seconds{1}),
+       std::chrono::seconds{1});
+
+  return true;
+}
+
+int main(int, const char**) {
+  test();
+  static_assert(test());
+
+  // test with the real tzdb
+  const std::chrono::tzdb& tzdb = std::chrono::get_tzdb();
+  assert(!tzdb.leap_seconds.empty());
+  test(tzdb.leap_seconds[0], tzdb.leap_seconds[0].value());
+
+  return 0;
+}
diff --git a/libcxx/test/std/time/time.zone/time.zone.leap/nonmembers/comparison.pass.cpp b/libcxx/test/std/time/time.zone/time.zone.leap/nonmembers/comparison.pass.cpp
new file mode 100644
index 0000000..ac8b780
--- /dev/null
+++ b/libcxx/test/std/time/time.zone/time.zone.leap/nonmembers/comparison.pass.cpp
@@ -0,0 +1,85 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: no-filesystem, no-localization, no-tzdb
+
+// TODO TZDB test whether this can be enabled with gcc 14.
+// UNSUPPORTED: gcc-13
+
+// XFAIL: libcpp-has-no-incomplete-tzdb
+// XFAIL: availability-tzdb-missing
+
+// <chrono>
+
+// class leap_second;
+
+//constexpr bool operator==(const leap_second& x, const leap_second& y);           // C++20
+//constexpr strong_ordering operator<=>(const leap_second& x, const leap_second& y);
+//
+//template<class Duration>
+//  constexpr bool operator==(const leap_second& x, const sys_time<Duration>& y);
+//template<class Duration>
+//  constexpr bool operator< (const leap_second& x, const sys_time<Duration>& y);
+//template<class Duration>
+//  constexpr bool operator< (const sys_time<Duration>& x, const leap_second& y);
+//template<class Duration>
+//  constexpr bool operator> (const leap_second& x, const sys_time<Duration>& y);
+//template<class Duration>
+//  constexpr bool operator> (const sys_time<Duration>& x, const leap_second& y);
+//template<class Duration>
+//  constexpr bool operator<=(const leap_second& x, const sys_time<Duration>& y);
+//template<class Duration>
+//  constexpr bool operator<=(const sys_time<Duration>& x, const leap_second& y);
+//template<class Duration>
+//  constexpr bool operator>=(const leap_second& x, const sys_time<Duration>& y);
+//template<class Duration>
+//  constexpr bool operator>=(const sys_time<Duration>& x, const leap_second& y);
+//template<class Duration>
+//  requires three_way_comparable_with<sys_seconds, sys_time<Duration>>
+//  constexpr auto operator<=>(const leap_second& x, const sys_time<Duration>& y);
+
+#include <cassert>
+#include <chrono>
+
+#include "test_macros.h"
+#include "test_comparisons.h"
+
+// Add the include path required by test_chrono_leap_second.h when using libc++.
+// ADDITIONAL_COMPILE_FLAGS(stdlib=libc++): -I %{libcxx-dir}/src/include
+#include "test_chrono_leap_second.h"
+
+constexpr void test_comparison(const std::chrono::leap_second lhs, const std::chrono::leap_second rhs) {
+  AssertOrderReturn<std::strong_ordering, std::chrono::leap_second>();
+  assert(testOrder(lhs, rhs, std::strong_ordering::less));
+
+  AssertOrderReturn<std::strong_ordering, std::chrono::leap_second, std::chrono::sys_seconds>();
+  assert(testOrder(lhs, rhs.date(), std::strong_ordering::less));
+
+  AssertOrderReturn<std::strong_ordering, std::chrono::sys_seconds, std::chrono::leap_second>();
+  assert(testOrder(lhs.date(), rhs, std::strong_ordering::less));
+}
+
+constexpr bool test() {
+  test_comparison(test_leap_second_create(std::chrono::sys_seconds{std::chrono::seconds{0}}, std::chrono::seconds{1}),
+                  test_leap_second_create(std::chrono::sys_seconds{std::chrono::seconds{1}}, std::chrono::seconds{2}));
+
+  return true;
+}
+
+int main(int, const char**) {
+  test();
+  static_assert(test());
+
+  // test with the real tzdb
+  const std::chrono::tzdb& tzdb = std::chrono::get_tzdb();
+  assert(tzdb.leap_seconds.size() > 2);
+  test_comparison(tzdb.leap_seconds[0], tzdb.leap_seconds[1]);
+
+  return 0;
+}
diff --git a/libcxx/test/support/test_chrono_leap_second.h b/libcxx/test/support/test_chrono_leap_second.h
new file mode 100644
index 0000000..485f68d91
--- /dev/null
+++ b/libcxx/test/support/test_chrono_leap_second.h
@@ -0,0 +1,52 @@
+// -*- C++ -*-
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#ifndef SUPPORT_TEST_CHRONO_LEAP_SECOND_HPP
+#define SUPPORT_TEST_CHRONO_LEAP_SECOND_HPP
+
+// Contains helper functions to create a std::chrono::leap_second.
+//
+// Since the standard doesn't specify how a @ref std::chrono::leap_second is
+// constructed this is implementation defined. To make the public API tests of
+// the class generic this header defines helper functions to create the
+// required object.
+//
+// Note This requires every standard library implementation to write their own
+// helper function. Vendors are encouraged to create a pull request at
+// https://github.com/llvm/llvm-project so their specific implementation can be
+// part of this file.
+
+#include "test_macros.h"
+
+#if TEST_STD_VER < 20
+#  error "The format header requires at least C++20"
+#endif
+
+#include <chrono>
+
+#ifdef _LIBCPP_VERSION
+
+// In order to find this include the calling test needs to provide this path in
+// the search path. Typically this looks like:
+//   ADDITIONAL_COMPILE_FLAGS(stdlib=libc++): -I %{libcxx-dir}/src/include
+// where the number of `../` sequences depends on the subdirectory level of the
+// test.
+#  include "tzdb/leap_second_private.h" // Header in the dylib
+
+inline constexpr std::chrono::leap_second
+test_leap_second_create(const std::chrono::sys_seconds& date, const std::chrono::seconds& value) {
+  return std::chrono::leap_second{std::chrono::leap_second::__constructor_tag{}, date, value};
+}
+
+#else // _LIBCPP_VERSION
+#  error                                                                                                               \
+      "Please create a vendor specific version of the test typedef and file a PR at https://github.com/llvm/llvm-project"
+#endif // _LIBCPP_VERSION
+
+#endif // SUPPORT_TEST_CHRONO_LEAP_SECOND_HPP