Excel date serial converter

Excel stores dates as days since 1 January 1900, with the fraction being the time of day. Serial 45244.5 is midday on 14 November 2023.

Excel believes 1900 was a leap year. It was not — a century year is only a leap year if divisible by 400, and 1900 is not. The bug was inherited deliberately from Lotus 1-2-3 for file compatibility and has never been fixed, because fixing it would change every existing spreadsheet. The practical effect is that serial numbers from 61 onward are one day ahead of a true day count, which is why converting via 1899-12-30 rather than 1900-01-01 gives correct answers for every date after 28 February 1900. Dates in January and February 1900 cannot be reconciled and are simply wrong in Excel. Separately, Excel for Mac historically used a 1904 system, so a file moved between platforms could shift by four years and a day.