If you want to determine what the difference is between two dates, you can use the DATEDIFF function. To use this function, indicate what segment of the date to compare, then provide the two dates. The segment of the date to compare comes from the intervals mentioned in the DATEADD function.
The syntax is:

SELECT DateDiff(interval, first date, second date)

SELECT DateDiff(week, '5/2/1998', '6/15/1998')

The difference is 7 weeks.
Now try what is the difference in

1. days?
2. Months?
3. Years?

You will quickly realize that the value returned is a rounded value, representing the nearest whole, integer value. So, for the number of months between the two dates, the result is 1.