Feature: Use real-time "wallclock" timekeeping units (#11341)

src/timer/timer_game_calendar.h

@@ -34,6 +34,8 @@ public:
 	static Date date; ///< Current date in days (day counter).
 	static DateFract date_fract; ///< Fractional part of the day.
 
+	static YearMonthDay ConvertDateToYMD(Date date);
+	static Date ConvertYMDToDate(Year year, Month month, Day day);
 	static void SetDate(Date date, DateFract fract);
 };