Unix timestamp conversion: Understanding seconds, milliseconds, and UTC time differences

Distinguish between seconds and milliseconds first

The reference point for Unix time is January 1, 1970, 00:00:00 UTC. In this tool, multiply the seconds value by 1000 to get the millisecond value. For example, the two numbers below represent the same moment. Since the unit is not automatically determined based on digits alone, check the unit description in the API or log format from which the number was received before selecting it.

1704067200 s
1704067200000 ms
2024-01-01T00:00:00.000Z

If you incorrectly select milliseconds for the seconds value above, you will get a date in 1970 instead of 2024. Even if there is no error message, the meaning may be incorrect. You should separately check whether the format is valid and when you intended it.

How to convert numbers and UTC dates

1. Choose Number → Date and choose seconds or milliseconds.
2. Enter an integer timestamp and press Calculate.
3. Check the results in UTC, device time, seconds, and milliseconds.
4. For the reverse conversion, select UTC date → number and enter in YYYY-MM-DDTHH:mm:ss format. If you need milliseconds, prefix .SSS.

The example date 2024-01-01T00:00:00 is interpreted as midnight UTC. If you want to enter midnight in Korean time, you must first consider the time difference from UTC. This tool does not have an optional feature to enter an arbitrary country time zone. If your device's time zone is set incorrectly, your device's time display may not be as expected.

Handling negative numbers, decimal points, and invalid dates

0 seconds is the reference point itself, -1 second is 1969-12-31T23:59:59.000Z. Numeric input does not accept decimal places. If you need 1.5 seconds, enter 1500 in milliseconds mode. The resulting seconds are milliseconds divided by 1000 and rounded down, so the integer second representation of -1 millisecond is -1.

In date input, dates that do not exist, such as February 30, are treated as errors. The supported range is from 0001 to 9999 and does not account for business days, holidays, legal deadlines, or leap seconds. If you just want to know the number of days between dates, a separate date calculator is simpler.

Time expression notes: MDN JavaScript Date. The implementation uses milliseconds as an internal reference and separates the ISO string from the device time representation.

Frequently Asked Questions