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Discussion on: Today I intentionally copied a bug

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Aaron Reese

There is a bug in Excel that results in incorrect leap year calculations. The bug was originally introduced in Lotus 123 and Microsoft intentionally copied the bug so as to be able to claim compatibility. Go figure....
The bug is that centuries should NOT be leap years even though they are divisible by 4 UNLESS they are also a millennium. 1896, 1904, 1996, 2000 and 2004 are leap years. 1900 was not. Fortunately for me it is unlikely to significantly impact my work in the run up to 2100.