Epoch Time Converter (Unix, FILETIME, Excel)

How to use

1. Pick a source format from the dropdown (default is ISO 8601 set to now).
2. Paste or type the value in that format — all 11 representations update instantly.
3. Switch the format dropdown to convert: the displayed value re-fills the box in that new format.
4. Use a preset to seed common reference moments: now, the Unix epoch (1970), Y2K, the Y2038 problem, or the NTP 32-bit rollover (2036-02-07).
5. Click any row's copy button to copy that representation to clipboard.

Frequently asked questions

Why does Excel's 1900 serial date count from 1899-12-30 instead of 1900-01-01?

Lotus 1-2-3 incorrectly treated 1900 as a leap year (it isn't — 1900 ÷ 100 = 19 with remainder 0 but 1900 ÷ 400 ≠ integer, so the Gregorian rule excludes it). When Microsoft built Excel they copied Lotus's bug for spreadsheet compatibility, which means Excel has a phantom February 29, 1900. To make the offset work cleanly for all dates after 1900-03-01 we use 25569 (1899-12-30 to 1970-01-01 in days), which gives correct results for every real date. Dates before 1900-03-01 in Excel are off by one day, but those are extremely rare in practice.

Why use BigInt for FILETIME and .NET Ticks?

FILETIME counts 100-nanosecond intervals from 1601-01-01 UTC. For a moment around year 2100 that's ~1.6 × 10¹⁷ — well past Number.MAX_SAFE_INTEGER (~9 × 10¹⁵). The bottom digits silently drift if you store it as a regular JavaScript number. Same for .NET ticks (~6.6 × 10¹⁷ around 2100). BigInt is JavaScript's arbitrary-precision integer type and keeps every digit exact, so round-tripping a FILETIME through this tool gives back exactly the same digits.

What's the Y2038 problem, and is your Unix output affected?

Many legacy C programs store Unix time as a signed 32-bit integer, which overflows at 2³¹ − 1 seconds past 1970 = 03:14:07 UTC on 2038-01-19. This tool stores time as JavaScript milliseconds (a 64-bit float, valid for ±285 614 years around 1970), so Y2038 isn't a problem here. We expose Y2038 as a preset so you can see how the formats represent that moment. NTP has its own rollover in 2036 because the 32-bit seconds counter restarts; that's also a preset.

Why both Excel 1900 and Excel 1904?

Excel for Windows uses 1900 as its base; Excel for Mac (and Numbers, until recently) uses 1904. A spreadsheet emailed between a Windows analyst and a Mac one will look correct in each app but will silently shift by ~4 years if the underlying serial numbers get copied raw. Both formats are exposed so you can detect or convert between them.

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