how to compare to datetimes in access by using query?

You would use the DateDiff operator. It takes three criteria. The first is the units you want the difference to be in (days, months, years, etc.) in the typical format for the units (dd, yyyy, etc.) The next is the date from your table and the last is the current date/time (either Date() for just the date or Now() for the date and time).

Here is a page with more info on this operator:

http://www.techonthenet.com/access/functions/date/datediff.php[^]