-/* Time functions */
-int is_leap_year(year_t year);
-int days_in_year(year_t year);
-int days_in_month(year_t year, month_t month);
-int weeks_in_month(year_t year, month_t month);
-wday_t day_of_week(year_t year, month_t month, day_t day);
-wday_t start_of_month(year_t year, month_t month);
-day_t start_of_week(year_t year, month_t month, day_t day);
-void add_days(year_t *year, month_t *month, day_t *day, int days);