Excel date to Unix timestamp.

The formulas

Excel stores a date as a day count with the time of day as a fraction. In the 1900 system, serial 25569 is 1970-01-01, so:

• Unix seconds = (serial − 25569) × 86400
• Excel serial = unix seconds / 86400 + 25569

In a cell holding a serial in A1: =(A1-25569)*86400. For a workbook in the 1904 system use 24107 instead of 25569; the two systems differ by 1,462 days. Microsoft says the 1900 system counts from 1 January 1900 and the 1904 system from 1 January 1904.

Excel's phantom leap day

The 1900 system treats 1900 as a leap year, which it was not, so serial 60 is a 29 February 1900 that never existed. That is why serials 1 to 59 are one day later than the plain formula gives, and why this tool refuses serial 60. From serial 61 (1 March 1900) the formula is exact.

Time zones

An Excel date has no time zone. The tool treats the serial as a UTC clock reading; if your sheet holds local time, subtract the zone offset in seconds afterwards.

FAQ

How do I convert an Excel date to a Unix timestamp?

Use (serial - 25569) * 86400 for the 1900 date system, or (serial - 24107) * 86400 for the 1904 system. The tool above does it and shows the date it lands on.

What is Excel serial 25569?

25569 is 1 January 1970 in the 1900 date system, the Unix epoch. In the 1904 system the epoch is serial 24107.

Why does Excel think 1900 was a leap year?

Lotus 1-2-3 treated 1900 as a leap year and Excel kept the behaviour for compatibility, according to Microsoft. Serial 60 is therefore a date that never existed.

Which date system is my workbook using?

Microsoft says newer versions of Excel use the 1900 system and older versions used the 1904 system, so a workbook from an old version may still be in 1904. A date such as 2011-07-05 is a different serial in each system, differing by 1,462 days.

What is Unix epoch time?

Unix time counts seconds since 00:00:00 UTC on 1 January 1970, the Unix epoch. POSIX defines it so that every day is exactly 86,400 seconds, which is why 1970-01-02T00:00:00Z is 86400.

Which time zone is a Unix timestamp in?

None. It identifies one instant, counted from the epoch in UTC. A time zone only matters when you turn the number into a calendar date and clock time, or the other way round.

Is my data sent anywhere?

No. The conversion runs in your browser and nothing you type is uploaded or stored.

Sources (each opened 2026-10): Microsoft Support, Date systems in Excel; Microsoft Learn, Excel incorrectly assumes that the year 1900 is a leap year; POSIX.1-2017 Base Definitions, 4.16 Seconds Since the Epoch.