I think Dec 15th is probably the "average" date - at least of the schools that I looked at.
I wouldn't be surprised if we see some attempt at standardization by the APA in the future, though I imagine getting schools to comply will be tricky. Its becoming increasing ridiculous though as everyone wants to be "first" so schools keep moving back their interview date. A few years ago, my school was always one of the first interviews people went on, now we're usually middle of the pack, if not on the tail end, and we've had roughly the same timeframe the whole way through.
Honestly, I can't say that I really understand it - I know there's probably a few people who are just looking for ANY acceptance and won't even bother visiting other places after receiving an offer, but I imagine they are in the minority and probably not the highest caliber student to begin with.
OP: You can probably send an updated transcript or unofficial grades if they desire, though as long as your GPA is decent, I doubt a semester's worth of grades is going to be the thing that makes or breaks your app. Typically its the research experience that holds back people applying straight out of undergrad - not the extra coursework.