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
Find the number of days between two specified dates
DATEDIFF('day',#2017-01-01# , TODAY())
DATEDIFF with DATETRUNC lets you track date differences without hard coding a specific date.