All databases have date manipulation functions built in. They are not entirely portable. Many people store a 'date' as unix epoch time which is a 4 byte int, easy to sort, easy to compare, easy to mamipulate. If you really want to use stringified dates and then perform essentially integer operations on them I would suggest using the RDBS to do the work for you but you will have portability issues. On SQL 7....

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 tr
+aded 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)
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See Compare Oracle and SQL dates as Oracle is different.