Use time_t instead of long in several places

Start converting some variables and return values to store times from
long to time_t.

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/ui-calendar.c

@@ -214,7 +214,7 @@ struct date *ui_calendar_get_slctd_day(void)
 }
 
 /* Returned value represents the selected day in calendar (in seconds) */
-long ui_calendar_get_slctd_day_sec(void)
+time_t ui_calendar_get_slctd_day_sec(void)
 {
 	return date2sec(slctd_day, 0, 0);
 }