Why is my timestamp 13 digits?

Unix time counts from midnight UTC on 1 January 1970, and each unit is a factor of 1,000 from the next. For any date between September 2001 and the year 2286, seconds take 10 digits. Milliseconds take 13, microseconds 16 and nanoseconds 19. So 1700000000 is 14 November 2023 at 22:13:20 UTC, and 1700000000000 is the same instant to the millisecond. The length is the tell. A 12-digit value is still milliseconds, from a date between 1973 and 2001.

parsetime applies the same rule. It reads the unit from the length of the number and names it above the result, so a millisecond value and a seconds value both come out as November 2023.

Pass a millisecond value to code that expects seconds and the date lands in the year 55840. Python's datetime.fromtimestamp(1700000000000) raises ValueError: year 55840 is out of range, and a database that accepts it stores a date tens of thousands of years out. Pass seconds where milliseconds are expected and you get the opposite: new Date(1700000000) in JavaScript is 20 January 1970 at 16:13:20 UTC, less than three weeks after the epoch.

Both symptoms are the same bug seen from either side. A date in January 1970 means the value was too small by 1,000. A date with a five-digit year means it was too large by 1,000.

JavaScript's Date.now() and Java's System.currentTimeMillis() return milliseconds, which is why 13-digit values fill logs, JSON payloads and browser code. Python's time.time() returns seconds as a float, so int(time.time()) is 10 digits. Go's time.Now().Unix() is seconds and time.Now().UnixMilli() is milliseconds. Ruby's Time.now.to_i and the shell's date +%s are seconds. PostgreSQL's extract(epoch from now()) is seconds with a fraction. Java's Instant.now().getEpochSecond() is seconds too, so one language can hand you both units.

When two systems disagree by a factor of 1,000, the one written in JavaScript or Java is usually the one sending milliseconds.

Divide by 1,000 to go from milliseconds to seconds, and keep the remainder if you need it: 1700000000123 is 1700000000 seconds plus 123 milliseconds. Integer division drops the fraction, so Math.floor(ms / 1000) in JavaScript and ms // 1000 in Python both give 1700000000. Multiply by 1,000 to go the other way.

Keep the value as an integer while you do it. A 32-bit float holds about seven significant digits, so a 13-digit count that passes through one comes back changed. A 64-bit double is exact up to 2^53, which covers every millisecond timestamp. If you only need the date, paste the number below: parsetime shows the seconds and milliseconds forms side by side.