fix: add quantity_point::origin, like std::chrono::time_point::clock

src/core/include/units/chrono.h

@@ -25,6 +25,7 @@
 #include <units/customization_points.h>
 // IWYU pragma: begin_exports
 #include <units/isq/si/time.h>
+#include <units/point_origin.h>
 #include <chrono>
 // IWYU pragma: end_exports
 
@@ -38,10 +39,13 @@ struct quantity_like_traits<std::chrono::duration<Rep, Period>> {
   [[nodiscard]] static constexpr rep number(const std::chrono::duration<Rep, Period>& q) { return q.count(); }
 };
 
+template<typename C>
+struct clock_origin : point_origin<isq::si::dim_time> { };
+
 template<typename C, typename Rep, typename Period>
 struct quantity_point_like_traits<std::chrono::time_point<C, std::chrono::duration<Rep, Period>>> {
-  using dimension = isq::si::dim_time;
-  using unit = downcast_unit<dimension, ratio(Period::num, Period::den)>;
+  using origin = clock_origin<C>;
+  using unit = downcast_unit<typename origin::dimension, ratio(Period::num, Period::den)>;
   using rep = Rep;
   [[nodiscard]] static constexpr auto relative(
     const std::chrono::time_point<C, std::chrono::duration<Rep, Period>>& qp) {
@@ -49,4 +53,11 @@ struct quantity_point_like_traits<std::chrono::time_point<C, std::chrono::durati
   }
 };
 
+namespace detail {
+
+template<typename C, typename Rep, typename Period>
+inline constexpr bool is_quantity_point_like<std::chrono::time_point<C, std::chrono::duration<Rep, Period>>> = true;
+
+}  // namespace detail
+
 } // namespace units