1 files changed, 36 insertions, 0 deletions
diff --git a/src/buildtool/system/system.hpp b/src/buildtool/system/system.hpp
index 2038d5c7..fc67471b 100644
--- a/src/buildtool/system/system.hpp
+++ b/src/buildtool/system/system.hpp
@@ -1,10 +1,46 @@
 #ifndef INCLUDED_SRC_BUILDTOOL_SYSTEM_SYSTEM_HPP
 #define INCLUDED_SRC_BUILDTOOL_SYSTEM_SYSTEM_HPP
 
+#include <chrono>
+
+#include "src/utils/cpp/concepts.hpp"
+
 namespace System {
 
 void ExitWithoutCleanup(int exit_code);
 
+/// \brief Obtain POSIX epoch time for a given clock.
+/// Clocks may have different epoch times. To obtain the POSIX epoch time
+/// (1970-01-01 00:00:00 UTC) for a given clock, it must be converted.
+template <class T_Clock>
+static auto GetPosixEpoch() -> std::chrono::time_point<T_Clock> {
+    // Since C++20, the system clock's default value is the POSIX epoch time.
+    std::chrono::time_point<std::chrono::system_clock> sys_epoch{};
+    if constexpr (std::is_same_v<T_Clock, std::chrono::system_clock>) {
+        // No conversion necessary for the system clock.
+        return sys_epoch;
+    }
+    else if constexpr (ClockHasFromSys<T_Clock>) {
+        // The correct C++20 way to perform the time point conversion.
+        return T_Clock::from_sys(sys_epoch);
+    }
+    else if constexpr (ClockHasFromTime<T_Clock>) {
+        // Older releases of libcxx did not implement the standard conversion
+        // function from_sys() for std::chrono::file_clock. Instead the
+        // non-standard function file_clock::from_time_t() must be used. Since
+        // libcxx 14.0.0, this has been fixed by:
+        //  - https://reviews.llvm.org/D113027
+        //  - https://reviews.llvm.org/D113430
+        // TODO(modernize): remove this once we require clang version >= 14.0.0
+        return T_Clock::from_time_t(
+            std::chrono::system_clock::to_time_t(sys_epoch));
+    }
+    static_assert(std::is_same_v<T_Clock, std::chrono::system_clock> or
+                      ClockHasFromSys<T_Clock> or ClockHasFromTime<T_Clock>,
+                  "Time point conversion function unavailable.");
+    return {};
 }
 
+}  // namespace System
+
 #endif  // INCLUDED_SRC_BUILDTOOL_SYSTEM_SYSTEM_HPP