I had such an aha! moment this afternoon that I thought I'd share it.
This test case was failing for me, and I was positive that I hadn't made any changes which would affect that part of the system:
The trick here is when I was running the test: March 9, 2013, at 1:55pm EST. Turns out that tonight is daylight savings, when we "spring ahead" and lose an hour, and of course 23 hours does not a standard day make!