/age doesn't give the exact age, only the number of months. I beleive TsunamiZ is asking if there is a way to determine the number of days his character has been around, probably to tell when its birthday is coming around.
i had the same problem with my warrior, im pretty sure he is asking, specifically how many days are left till his birthday, 11 months doesnt tell you if its 1 day till 12 or 30 days till 12 months, so, cut the guy a break why dont ya
Yeah, there's no easy way of finding out the exact day of the month that your character was created, but checking your /age every day until your character hits 1 year would work, although time consuming.
Sadly, the only way for now that I've seen is just checking every day when you're at 11 months. Once it changes to 12, that's you B-Day. That's what I did for my main's B-Day, but I know how little that actually helps. I just hope your GW.dat isn't super huge, that could help a lot.