Find the difference between two dates using DATEDIFF
DATEDIFF(date_part, start_date, end_date, [start_of_week])
The date_part can be:
[start_of_week] is an optional parameter which can be any day of the week (i.e "Monday", "Tuesday", etc)
Find the number of months between two specified dates
DATEDIFF('month',#2017-07-01#, #2017-09-01#)
Find the number of days between two specified dates
DATEDIFF('day',#2017-01-01# , TODAY())
Some tips:
DATEDIFF with DATETRUNC lets you track date differences without hard coding a specific date.