📅Microsoft Tools
Excel Date Serial Converter
Paste an Excel serial number (like 45658) to get the real date, or pick a date to get its Excel serial. Handles Excel's notorious Feb 29, 1900 leap-year bug correctly. Great for debugging imported spreadsheet data where dates appear as plain numbers.
How Excel date serials work
January 1, 1900 = 1. Each day adds 1. Excel has a known bug where it counts February 29, 1900 as a real day (it wasn't a leap year), so serials above 59 are offset by 1.