Convert Excel's serial date numbers (e.g. 45292) to real dates and back. Decimal fractions are converted to time of day.
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Serial number → date
Date → serial number
How to use
Serial → date: type the number Excel shows (e.g. 45292) to see the real date.
Decimals (e.g. 45292.5) are converted to a time of day.
Date → serial: pick a date (and time) to get the serial number.
Serial dates and the 1900 leap-year bug
Excel stores dates internally as a running count of days where January 1, 1900 = 1; if a cell's format is set to Number you see this raw count instead of a date. The fractional part is the time of day (0.5 = noon). Excel also famously treats the non-existent date February 29, 1900 as serial 60 — this tool accounts for that quirk, flags serial 60 explicitly, and converts everything from March 1, 1900 onward correctly.
FAQ
A spreadsheet from a Mac shows dates about 4 years off.
Old Mac versions of Excel used a 1904-based date system, shifting everything by 1,462 days. Add 1462 to the number and convert it here to recover the real date.