As this is the programming sub-forum, I have an amusing story to tell you. It was when I was doing Y2K testing on a sales order processing suite. I asked it to do me a 29-02-2000 month end and it gave me the "invalid date" message.................
I did what a beancounter would have done in the situation, and asked for 28-02-2000, because the 29th was actually a non-trading day for us, and even if it wasn't I had a work around by delaying the month-end run and processing all the 29th stuff as the 28th ;)
I fell apart laughing when the message popped up "February 28th is not a valid month-end, would you like to use February 29th?"