Indeed they do. The hardest bugfix I ever had to do (which I go back to whenever interviews ask that question) was a timezone bug that was going to ruin the iOS 7 on-stage reveal. It only reproduced in customer builds and customer hardware, which we could not debug or get root on. Anyway we traced it to a confluence of rare conditions in a date-math C file that had not been modified since 1987, and was in a completely different style from everything else in the OS. The edge-iest of edge cases.

It was hard because of the debugging blindness, not so much date-math itself. We had to do hardware-level things.