Well, since our release schedule is two months out, I've been writing 2007 for two months. My problem is remembering what the current month is, letting along the day.
The way it generally works with me is that I'm fine for the first 6-8 weeks, because I'm being a little careful. And then, once we're "into the year" my vigilance slips, and lo and behold I'm dropping previous year dates at the end of February, when everyone else is over the switch!