I discovered this one, recently: > Excel stores dates as sequential serial numbers so that they can be used in calculations. By default, January 1, 1900 is ser

I discovered this one, recently: > Excel stores dates as sequential serial numbers so that they can be used in calculations. By default, January 1, 1900 is serial number 1, and January 1, 2008 is serial number 39448 because it is 39,447 days after January 1, 1900. That's from the Microsoft's documentation of Excel[1]. Note this part: "because [Jan 1, 2008] is 39,447 days after January 1, 1900" In [3]: (datetime.datetime(2008, 1, 1) - datetime.datetime(1900, 1, 1)).days Out[3]: 39446 Off by one, Microsoft. And yes, Excel does the same thing — i.e., the docs match the product. If you're wondering why, it's b/c Excel incorrectly thinks that 1900 is a leap year. (Also, this is why a lot of Window's clocks track time since 1901, as it makes leap year calculations simpler… for now.) [1]: https://support.office.com/en-us/article/DATEVALUE-function-df8b07d4-7761-4a93-bc33-b7471bbff252 https://support.office.com/en-us/article/DATEVALUE-function-...