Well, at least if you're me.
I found some bugs in the DateTime::TimeZone code. I spent large chunks of the last two days pounding my head against the awful code I'd written to parse the Olson time zone database and calculate time zone changes. My brain hurts.
On the plus side, most of the bugs are now fixed, and the code is a bit cleaner, so next time I have to look at it it won't be quite so painful.