A small trivia piece: all Excel dates start at New Year's Eve 1899, not January 1st 1900. Excel considers the year 1900 as a leap year, which it was not.
This is a bug introduced by Microsoft on purpose for reasons of compatibility with Lotus 1-2-3, a popular pre-windows spreadsheet tool.
Be mindful of this when adding serial dates originating from Excel to a start date in a different system (such as R).