I've already written once about Daylight Saving and how much I hate it. It came back to bite me anyway.
I changed the scripts to run at 15,16 in cron (11am and 12pm respectively). I added in some Perl code that would detect and switch to the appropriate timezone. Little did I know that MySQL (at least our installation) doesn't have an EDT timezone. So everything relating to it has been silently failing.
"Luckily", MySQL has a timezone just called "US/Eastern", which apparently is correct whether you're in Daylight Saving mode or not (though I question how queries using that with forward/backward dates would work).