Codechange: move all date-related variables inside the timer (#10706)

src/toolbar_gui.cpp

@@ -1149,8 +1149,8 @@ void SetStartingYear(Year year)
 	_settings_game.game_creation.starting_year = Clamp(year, MIN_YEAR, MAX_YEAR);
 	Date new_date = ConvertYMDToDate(_settings_game.game_creation.starting_year, 0, 1);
 	/* If you open a savegame as scenario there may already be link graphs.*/
-	LinkGraphSchedule::instance.ShiftDates(new_date - _date);
-	SetDate(new_date, 0);
+	LinkGraphSchedule::instance.ShiftDates(new_date - TimerGameCalendar::date);
+	TimerGameCalendar::SetDate(new_date, 0);
 }
 
 /**