A Unix timestamp is an offset from an epoch, expressed in a unit. The number alone does not say whether the producer meant seconds or milliseconds. A digit-count heuristic can be convenient for recent dates, but it is not a reliable data contract. The Explorer never uses one.
Start with a known instant
Input: 1780496820
Input unit: Seconds
Timezone: UTC
Expected ISO: 2026-06-03T14:27:00.000Z
Expected Unix milliseconds: 1780496820000
Expected weekday: WednesdayRun the explicit-seconds example →
Change the input unit to milliseconds without changing the number. You have now described a different instant, much closer to January 1970. The tool follows the unit you selected. It does not override your choice because the year looks surprising.
The normalized model is milliseconds
The Explorer normalizes all supported inputs to an integer number of Unix epoch milliseconds. Seconds accept up to three decimal places, while milliseconds must be integers. The normalized value drives every output together, so the ISO date and numeric outputs cannot drift because separate conversions used different rounding rules.
Input: -0.125 seconds
Expected milliseconds: -125
Expected ISO: 1969-12-31T23:59:59.875Z
Input: 1780496820123 milliseconds
Expected seconds: 1780496820.123
Expected ISO: 2026-06-03T14:27:00.123ZExtra precision is rejected instead of rounded. Scientific notation, Infinity and NaN are not accepted. The supported Date range is ±8,640,000,000,000,000 milliseconds. Leap seconds are outside the model, as they are in JavaScript’s Date representation.
UTC describes an instant; local time adds clock rules
Choose UTC when you need a portable reading or are comparing logs from several regions. Browser local time uses this device’s timezone rules, and the result includes its zone name and offset. Changing the display timezone changes the clock reading, not the Unix value.
Calendar input is different: its timezone determines which instant a wall-clock reading denotes. Enter YYYY-MM-DDTHH:mm:ss, optionally followed by .SSS, and let the timezone selector supply the zone. Do not append Z or a numeric offset to that input. Use UTC for a source that already defines a UTC clock reading.
Some local clock readings are not unique
Daylight-saving transitions can skip a local time or repeat it. The Explorer rejects nonexistent times and detects repeated readings around common daylight-saving transitions rather than choosing an occurrence silently. For unusual historical timezone transitions, use a known UTC instant and verify the source offset separately. Impossible calendar dates are also rejected.