Excel is never ever going to break backwards compatability. In fact, quite some “features” in Excel are just there to stay bug-for-bug compatible with existing systems.

Example: Excel stores dates internally as a float - called the serial date, you can view it by running DATEVALUE on any cell that contains a date. It is supposed to be the number of days since 1 January 1900. However, since early Excel versions had to be compatible with Lotus1-2-3, Excel had to be compatible with a bug in Lotus123: they had erroneously assumed 1900 to be a leap year. In addition, the indexing is off by one. So the actual 0 epoch of an Excel serial date is 30 December 1899 for all dates starting 1 March 1900.