Seconds or milliseconds? How to tell which unit a Unix timestamp uses

Seconds and milliseconds count the same moments on different scales, so the digit count follows the unit. From 2001 to 2286, every seconds value has 10 digits and every milliseconds value has 13. Current values all fall in that span, so a number with any other length is either a finer unit, like microseconds, or a bug. The count has one blind spot. Millisecond values for dates from January 12, 1970 to April 26, 1970 also have 10 digits, so a number with 10 digits can be either unit.

Read a millisecond value as seconds and the date lands in year 58392. Depending on the tool, you'll often get an invalid date. Read a seconds value as milliseconds and it lands in 1970: new Date(1780511400) gives January 21, 1970 at 14:35:11 UTC.

When you type a whole number with at least 12 digits and the unit is set to seconds, the converter switches to milliseconds and tells you so. A seconds value that long would land between the years 5138 and 33658, so the converter assumes milliseconds. To read a number with fewer digits as milliseconds, or a long one as seconds, set the unit by hand.

To convert, multiply seconds by 1000 to get milliseconds, and divide milliseconds by 1000 and round down to get seconds. Rounding down matters at the edge. 1780511400999 still counts as second 1780511400, because its last three digits are the fraction of a second. Rounding to the nearest second would give the next one.

Common functions don't agree on the unit, so check each one's docs. JavaScript's `Date.now()` returns milliseconds, and PHP's `time()` returns seconds. Python's `time.time()` returns seconds as a floating-point number, so it can carry a fraction. Go's `Unix()` returns seconds, `UnixMilli()` returns milliseconds, and `UnixNano()` returns nanoseconds. Google Sheets' `EPOCHTODATE` takes seconds, milliseconds or microseconds, and its unit argument says which: 1 for seconds (the default), 2 for milliseconds and 3 for microseconds.