YEARFRAC

start_date: starting date serial value

end_date: ending date serial value

basis: calendar basis

YEARFRAC calculates the number of days from start_date to end_date according to the calendar specified by basis, which defaults to 0, and expresses the result as a fractional number of years.

If basis is 0, then the US 30/360 method is used. If basis is 1, then actual number of days is used. If basis is 2, then actual number of days is used within a month, but years are considered only 360 days. If basis is 3, then actual number of days is used within a month, but years are always considered 365 days. If basis is 4, then the European 30/360 method is used.

DATE.