Why is date arithmetic seemingly so hard for people? In the next 12 months there’s just 4 simple special cases to handle (switch to DST on March 29, Leap second on June 30 (I guess most of you didn’t know about that one), switch from DST on October 25, and leap year on February 29 2016)… Guess which of the four cases wasn’t accounted for in a unit test, which therefore “inexplicably” fails now even though it didn’t earlier.