You're either brave or foolhardy -- asking a Virginian how long it's going to take to drive from Maryland! I've held off answering in the hope that someone from MD who knows more would respond. Doesn't look like that's going to happen.

Thus, take my comments with some skepticism. Without traffic, it'd probably take about 45 minutes to drive to Clarendon from Germantown. I honestly don't know enough about traffic on I-270 to even guess what it'll be like during the afternoon rush on a Tuesday. I know the I-270 split (where part goes east and part goes south) can get bad because you lose lanes. You'll get on the Beltway to cross into Virginia. It will be very crowded, but should be moving (maybe about 30-40 mph?). You'll be mostly driving against traffic on the GW Parkway, so may be able to drive 45+ mph. Still, it's only 2-lane and if there's a problem, it can come to a complete stop. Once you exit the Parkway, the surface streets will be slow, but should move.

If I were driving from Germantown, I'd plan for at least 1 hour 15 minutes, but who knows how far off the mark I'd be. Still, hope you can make it Tuesday the 6th.