Calculates the difference in completed months between the start date and end date.

MONTHDIFF(StartDate, EndDate)

StartDate: the earliest date in the period being calculated.

EndDate: the latest date in the period being calculated.

If start date is greater than end date, 0 is returned

MONTHDIFF(DATE(1,1,2015), DATE(1,1,2016)) equals 12

MONTHDIFF(DATE(1,1,2015), DATE(1,31,2015)) equals 0