Codechange: Add a DateToYear function instead of dividing each time (#11173)

src/date_type.h

@@ -63,6 +63,16 @@ static constexpr TimerGameCalendar::Date DateAtStartOfYear(TimerGameCalendar::Ye
 	return (DAYS_IN_YEAR * year) + number_of_leap_years;
 }
 
+/**
+ * Calculate the year of a given date.
+ * @param date The date to consider.
+ * @return the year.
+ */
+static inline TimerGameCalendar::Year DateToYear(TimerGameCalendar::Date date)
+{
+	return date / DAYS_IN_LEAP_YEAR;
+}
+
 /**
  * The date of the first day of the original base year.
  */