feat(clock): support chrono Time Zone extensions.

Use chrono Calendars and Time Zones (P0355R7, P1466R3) when available
instead of the `date` library.
Verified with a patched build of a recent GCC 13 snapshot.

include/factory.hpp

@@ -1,7 +1,7 @@
 #pragma once
 
 #include <json/json.h>
-#ifdef HAVE_LIBDATE
+#if defined(HAVE_CHRONO_TIMEZONES) || defined(HAVE_LIBDATE)
 #include "modules/clock.hpp"
 #else
 #include "modules/simpleclock.hpp"

include/modules/clock.hpp

@@ -1,8 +1,7 @@
 #pragma once
 
-#include <date/tz.h>
-
 #include "ALabel.hpp"
+#include "util/date.hpp"
 #include "util/sleeper_thread.hpp"
 
 namespace waybar::modules {

include/util/date.hpp

@@ -1,8 +1,34 @@
 #pragma once
 
-#include <date/tz.h>
 #include <fmt/format.h>
 
+#if HAVE_CHRONO_TIMEZONES
+#include <chrono>
+#include <format>
+
+/* Compatibility layer for <date/tz.h> on top of C++20 <chrono> */
+namespace date {
+
+using namespace std::chrono;
+
+namespace literals {
+using std::chrono::last;
+}
+
+inline auto format(const std::string& spec, const auto& ztime) {
+  return spec.empty() ? "" : std::vformat("{:L" + spec + "}", std::make_format_args(ztime));
+}
+
+inline auto format(const std::locale& loc, const std::string& spec, const auto& ztime) {
+  return spec.empty() ? "" : std::vformat(loc, "{:L" + spec + "}", std::make_format_args(ztime));
+}
+
+}  // namespace date
+
+#else
+#include <date/tz.h>
+#endif
+
 template <typename Duration, typename TimeZonePtr>
 struct fmt::formatter<date::zoned_time<Duration, TimeZonePtr>> {
   std::string_view specs;