Excel Date Serial Number Converter
Turn a broken Excel date number (like 45123) back into a real date, and back again.
Exporting a database from Excel, or moving data between Excel and another program, often turns dates into a number like 45123 (a serial number). This page converts that number to a date, and a date back to the number, both ways. Paste a whole column copied from a spreadsheet to convert many rows at once. Nothing you enter leaves this screen.
Windows Excel counts 1 January 1900 as serial number 1, and carries an old bug that wrongly treats 1900 as a leap year, so every date from 1 March 1900 onward includes that bug. The old macOS Excel default, the 1904 system, counts 1 January 1904 as 0 and has no such bug. Which one your file uses depends on Excel's File > Options > Advanced > "Use 1904 date system" setting. This page cannot see that setting, so you choose it yourself.
Nothing you entered was sent anywhere. Everything was calculated on this screen.
This conversion reproduces Excel's actual behavior
The 1900 system deliberately reproduces Excel's leap year bug (wrongly treating 1900 as a leap year). Correcting that bug would make this tool disagree with real Excel by a day, which would make it wrong in practice. This page cannot tell whether your file uses the 1900 or 1904 system, so check that in Excel yourself.
Sources
How Excel date serial numbers and the 1900 bug workMade alongside
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