In general you would use function today() or now(), depending on the level of detail you need. But you need to define what a "month" is--is it 30 days? Is 02/13/2012 - 01/02/2012 1 month or 2?
The result of subtracting 2 dates will be an integer (possibly with decimal precision). You would then need to decide how to use that integer to define the # of months.
If you just want to test for between 6 and 12 months then I think my solution should work off you. If you want an accurate calculation then Rob Wunderlich suggested a very nifty solution on a previous thread: