The image above shows the DATEDIF function calculating the number of complete months between two dates. Column B and column C contains random dates.
Formula in cell D3:
Copy cell D3 and paste to cells below. The DATEDIF function requires the start date to be less than (earlier than) the end date, the function returns #NUM if not. It is the third argument that determines which unit to use. "M" is the abbreviation for the month unit.
Note that the DATEDIF function does not appear when you start typing the function, however, it is there don't worry. The same thing happens when you start typing the arguments, you get no guidance from Excel.