Added date functions to Spreadsheets

Added DATEDIF with the units Y, M, D, MD, YM, and YD; DAYS; WEEKNUM with Sunday, Monday, and ISO week types; ISOWEEKNUM; and YEARFRAC with day-count bases 0 to 4. Date text must be an unambiguous ISO date. Fixed MINUTE and SECOND returning one less than expected for some times created with TIME.