Eh, even though this topic is closed, may as well keep the zune reply in this one as opposed to being a reply-to-nothing in the new topic.
Unless you plan on checking every single date in the next X time, you only check the ones that appear relevant and potentially a random scatter. And the only relevant dates as far as leap years are concerned are February 29 and March 1.
This should have been caught in review, not testing.
Umm...I think you underestimate the budget of QA testing. It's fairly standard to test every relevant date within a certain range. Assuming it takes a tester 1 minute to change dates, checking every date for the next two years is approximately 12 hours of work (which is to say, one day's work in the life of a tester, so probably costs Microsoft about $100). Seriously, we had a tester collect 2160 collectible spider tokens in Spiderman: Web of Shadows--testing 750 dates is comparatively tame.
What I do wonder, however, is that from the description it sounds like the bug kicks in when you boot up the Zune; would the tester have missed the bug if he didn't turn the Zune on and off after every date switch?