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A classic bug for a leap day: At work, colleagues discovered and fixed a bug in a lesser used tool that only occurs on February 29th. It did trip up the tests, so no one got to merge until it got identified and resolved.

The source file in question hadn't been touched in over 4 years. Last leap year, the 29th was on a Saturday, so likely no one noticed.

What was annoying is that the logic had been clearly written with the intent of handling leap years. A leap year check condition was present, validated only on a February > 28th, but had to get inverted.

You may wonder why we wouldn't have used the languages' standard library date functions to validate the date - unfortunately the language in question doesn't have such a sophisticated standard library (language omitted to protect the innocent).