If there's a dispute over the date you moved out, then take whatever evidence you have of your actual move-out date with you to court. But if the landlord returned all but $250 of the deposit, and made no claim for rent at that time, it seems like a pretty weak claim to try to make now.
Have they actually countersued you? If not, and you have all but $250 of your deposit, the remaining question appears to be only whether that $250 was properly deducted from your deposit.

