Unix Timestamp Converter

What a Unix timestamp is

It is the number of seconds since midnight on 1 January 1970 in UTC. Its virtue is being a single number, with no time zone and no date format to interpret — which is why it turns up almost everywhere, from logs to authentication tokens to web interface fields.

Seconds or milliseconds

This is the ambiguity that wastes the most time. Many languages use seconds, but JavaScript and Java work in milliseconds. A millisecond value read as seconds lands roughly fifty thousand years in the future; a second value read as milliseconds lands back in 1970. Here the unit is inferred from the length of the number, which is the practical rule everyone uses.

Time zones

A timestamp is always UTC: it carries no time zone information at all. The date you get from it therefore depends on where you are, and that conversion happens in your browser, which is the only place that knows. UTC is shown too — that is the one to compare against server logs.

The year 2038

Systems storing the timestamp in a signed 32-bit integer overflow on 19 January 2038. The limit still exists in older software and in some embedded systems: if you work with dates far in the future, it is worth checking how many bits the receiving system uses.

How to use it

Paste the number — ten digits for seconds, thirteen for milliseconds — and the date arrives in UTC and your time zone, together with the ISO 8601 form to paste into documents. When the date lies in the future, the tool also shows how long is left, which is the right figure for telling whether a token has expired. In the other direction, picking a date and time yields the timestamp to use in the API or the query.

When it comes in handy

Reading an error log whose dates are bare numbers, checking the expiry of a JWT — which is a timestamp in disguise — filling a field that wants its date in seconds, or figuring out whether that number in a data export has already passed. In all these cases mental arithmetic is no match for copy and paste.