Understood. Right now you have to do mental time-arithmetic (the battle-launched time is in Eastern), but I had thought about that the other night, too -- it would be nice to have something that just told you "there are this many hours and seconds before lockdown". I'll work on that.
Also know that the 18-hour window is not absolute; whether or not reinforcements can jump in or not, is currently based on the battle status, which is not set until I run the forcedecs (which is manual, at this time, but will be automated soon). So the 18-hour window is a minimum; if someone were to launch an attack right after I ran the forcedecs on, say, a Friday, then you could jump in reinforcements until I run them again on Saturday (so it could possibly be as much as 24 hours in reality).
I will announce the change to an automated forcedec-generation system once I have one working, but for now, it's still manual.