You can use the DATEDIFF function. 

SELECT DATEDIFF(d, @HireDate, getdate()) 

d is a date part indicator for the day 

@HireDate can be an inserted parameter (like with a stored proc) or traded out for a column in a regular select expression. 

getdate() is a function that gets the current date.

You can do non platform independent SQL like:

SELECT id, store_date
FROM some_table
WHERE store_date >= date_add(now(), interval -3 month)