How to use Excel date
- 1. Paste a serial. 44927 is 2023-01-01 in the 1900 system.
- 2. Read ISO-8601. UTC midnight of that serial.
- 3. Or paste ISO → serial. Fractional time is supported.
- 4. Not a spreadsheet. No SheetJS workbook parse.
About this tool
Excel stores dates as day counts from a 1899-12-30 epoch on Windows. This converter maps that serial to ISO-8601 and back so CSV dumps can meet APIs that want timestamps.
1904 Mac system
Not implemented. Mac workbooks that use 1904 will be off by ~4 years.
Lotus leap day
Excel treats 1900 as a leap year. This converter does not emulate that bug.
Then convert to Unix time
After you get ISO-8601 from a serial, use the Unix timestamp converter for epoch seconds and milliseconds — handy when an API expects Unix time instead of Excel’s day count.
Code examples
Serial
44927