Transform

Convert raw Excel serial numbers like 45570 into human-readable YYYY-MM-DD dates

Translate cryptic Excel 5-digit serial numbers back into human-readable calendar dates.

🛡️ Your rows never leave your browser.

Why Onfyt: Accurately accounts for the historical Excel 1900 leap year bug to prevent dates from being off by exactly one day.

Interactive Sample Data

id	invoice_serial	amount
1	45570	1200.00
2	45571.5	850.50
3	45600	3400.00

How this tool works

Anyone who has exported data from Microsoft Excel to CSV, Python, or SQL has encountered the dreaded 5-digit integer: instead of seeing '2024-10-05', the exported file contains '45570'. If the cell included a time component, it looks like '45570.5'. This occurs because Excel does not store dates as text strings. Under the hood, Excel represents dates as serial numbers: the number of days elapsed since January 1, 1900. When Excel exports a spreadsheet without explicit text formatting applied, it emits the raw underlying numeric serial. Downstream systems like PostgreSQL or Python treat this as an integer, breaking date filters and dashboards. Furthermore, converting Excel serial numbers requires accounting for the infamous 'Excel 1900 leap year bug': Excel intentionally replicated a Lotus 1-2-3 bug that treated 1900 as a leap year, adding a phantom day (February 29, 1900) that never existed. DataFix handles this historical offset accurately, converting both whole serial days and fractional time components into clean ISO 8601 dates.

Things that usually go wrong

The Excel bug: Excel treats 1900 as a leap year. Serial 60 maps to the non-existent 1900-02-29. Serials below 61 are rare but handled specially.
Serial 0 converts to 1900-01-00 in Excel; the tool converts it to 1899-12-30 (the correct base).

Edge cases we tested

🧪 Fractional serial values representing time ('45570.75')

Converted to date plus exact clock time ('2024-10-05 18:00:00').

🧪 Serial values prior to March 1, 1900

Handled with exact epoch boundary math.

🧪 Mac Excel 1904 date system serials

Selectable toggle to convert from the 1904 epoch used by legacy Mac spreadsheets.

Frequently Asked Questions

What is an Excel date serial?

Excel stores dates as integers counting days since 1899-12-30 (with a historical leap-year bug). Serial 45570 = 2024-10-05.

Can I convert dates back to serials?

Yes — use the Date to Excel Serial tool.

What is the Excel epoch and why is it unusual?

Excel counts days from 1899-12-30 rather than 1900-01-01, due to a historical bug that incorrectly treated 1900 as a leap year. This means serial 1 = 1900-01-01, serial 60 = the non-existent 1900-02-29, and serial 61 = 1900-03-01.