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Unix timestamp: seconds vs milliseconds

Unix time counts seconds since 1970-01-01 UTC. Many JavaScript APIs use milliseconds instead. Mixing them shifts dates by about 50 years.

Quick tell

In JavaScript, Date.now() and date.getTime() are milliseconds. exp in a JWT is seconds.

10 digits (~1.7e9 today) → almost certainly seconds.
13 digits (~1.7e12) → almost certainly milliseconds.
16–19 digits → microseconds or nanoseconds (less common in web APIs).

Conversions

# seconds → ms
echo $((1700000000 * 1000))

# ms → seconds
echo $((1700000000000 / 1000))

# JS
new Date(1700000000 * 1000).toISOString();
Math.floor(Date.now() / 1000);

Leap seconds and time zones

Unix time ignores leap seconds. Display in a time zone is separate from the integer itself. The converter shows UTC and your local zone.

Questions

Why is my date in 1970?: You passed seconds to an API that expects milliseconds (or the reverse). Multiply or divide by 1000.
Is epoch always UTC?: The instant is UTC-based. Formatting it in Sydney or New York is a display choice.

See also

JWT decode vs verify: reading claims is not trusting them

Related tool: Unix timestamp converter. Runs in your browser; input is not uploaded.