I wrote and shipped some software once that worked fine for almost 6 months, then on April 15th (as I recall) started failing for no apparent reason in Korea only. It turned out to have a bug with Daylight Savings Time in the eastern hemisphere. That hurt. It gave me a heck of a lot of respect for calendar code ever since - I try to minimize it and then test the heck out of what's left.
I wrote and shipped some software once that worked fine for almost 6 months, then on April 15th (as I recall) started failing for no apparent reason in Korea only. It turned out to have a bug with Daylight Savings Time in the eastern hemisphere. That hurt. It gave me a heck of a lot of respect for calendar code ever since - I try to minimize it and then test the heck out of what's left.