in general, you'd likely use something like DATEDIFF
DATEDIFF (Transact-SQL)[
^] - but as Ive indicated in the comments, you need to provide more information - do you want the difference in seconds, days, months, ??? for example
a general form of SQL could be
select DATEDIFF(day, column1, column2)
from tablex
where column1 >= startdaterange AND
column2 <= enddaterange;