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Lotus 1-2-3 used an epoch from 1900-01-01, but it had a bug where it considered all years divisible by 4 to be leap years.

When Excel came around, it needed to be compatible with 1-2-3, so it used the same date format, and to be compatible it considers 1900 to be a leap year.

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Excel has two date systems, with either a 1900 or 1904 start date:

https://support.microsoft.com/en-us/office/date-systems-in-e...

The original Excel was for the Macintosh and had the 1904 date system, and was later brought to Windows where the 1900 date system was used.

Here’s a great story about the ongoing impact of those decisions. https://www.joelonsoftware.com/2006/06/16/my-first-billg-rev...

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