Excel 1900 leap year bug

I think that Excel's 1900-02-29 bug should be fixed. Over the years, many other users have complained about this.

• To prevent any disruption, "Thu, 1900-03-01" should retain its "day 61" serial number, so that all later dates remain similarly unchanged. In particular, "Mon, 2021-05-31" should retain its "day 44347" serial.
• "Wed, 1900-02-29" is INVALID and should be eliminated.
• "Wed, 1900-02-28" (not Tue!) should get a new "day 60" serial, ...
• ... and so on down to "serial 1" for "Sun, 1899-12-31".

Admittedly, this change could affect some existing Workbooks, but only for January 1900 and February 1900. Those dates are invalid anyway.

Excel now supports the totally useless "Thu, 2176-07-04", "Thu, 2576-07-04", and "Thu, 9776-07-04".
But not "Thu, 1776-07-04", even though the UK and her colonies switched to Gregorian in 1752.
So it may make sense to extend the serial number into negative territory, to cover the 19th century and earlier. That would help professional historians:

• "Sat, 1899-12-30" gets "day 0" serial.
• "Fri, 1899-12-29" gets "day -1" serial.
• "Thu, 1899-12-28" gets "day -2" serial.
• "Wed, 1899-12-27" gets "day -3" serial.
• ...
• "Thu, 1776-07-04" gets "day -45103" serial.
• etc. as far down as practical (always assuming the Gregorian calendar).

As indicated in https://video2.skills-academy.com/en-us/office/troubleshoot/excel/1900-and-1904-date-system , Excel already supports 2 calendar date systems:

• The 1900 Date System: preferably with the corrections proposed above.
• The 1904 Date System: The first day that is supported is January 1, 1904. Because of the design of early Macintosh computers, dates before January 1, 1904, were not supported.

It might be nice to add a 3rd choice:

• The .NET scheme described in https://video2.skills-academy.com/en-us/dotnet/standard/datetime/working-with-calendars , or at least the Gregorian scheme described in https://video2.skills-academy.com/en-us/dotnet/api/system.globalization.gregoriancalendar?view=net-5.0