Epoch Converter

Unix time, and why dates land in 1970

Unix time counts the seconds elapsed since 1 January 1970 UTC. It is a single integer with no time zone attached, which makes it ideal for storage and comparison — and a common source of off-by-a-thousand bugs.

How it works

• Converts between an integer timestamp and a human-readable date in UTC and your local zone.
• Detects the unit from the magnitude: a 10-digit value is seconds, 13 digits is milliseconds, 16 is microseconds, 19 is nanoseconds.
• Ignores leap seconds, as Unix time itself does — a Unix day is always exactly 86,400 seconds.

seconds since 1970-01-01T00:00:00Z

1 000 000 000    →  2001-09-09   (10 digits, seconds)
1 700 000 000    →  2023-11-14   (10 digits, seconds)
1 700 000 000 000 →  2023-11-14  (13 digits, milliseconds)

2^31 − 1 = 2 147 483 647 → 2038-01-19T03:14:07Z

Worked example

The classic bug: a JavaScript timestamp handed to something expecting seconds.

1. Date.now() returns 1750000000000 — milliseconds, 13 digits
2. passed to a system expecting seconds, it reads as 1,750,000,000,000 seconds
3. that is roughly the year 57,442 — or, if the field overflows, 1970
4. the fix: Math.floor(Date.now() / 1000) → 1750000000

Whenever a date comes out as 1970 or as an absurd far-future year, the cause is almost always this factor of 1,000. Count the digits before debugging anything else.

Reading the result

• A timestamp has no time zone. Rendering it as a local date is a display decision; the stored integer is always UTC. Two users seeing different dates for the same timestamp is correct behaviour, not a bug.
• Systems storing seconds in a signed 32-bit integer overflow on 19 January 2038. Anything with a long lifetime should use 64-bit.
• Negative timestamps represent dates before 1970 and are valid, but handling varies — some platforms and databases reject or mishandle them.
• Because leap seconds are ignored, Unix time is not a true count of elapsed SI seconds since the epoch. For almost everything this does not matter; for precise interval timing, use a monotonic clock instead.

Common questions

Why does my date show as 1 January 1970?

The value reaching the converter was zero, null, or an unparsed string that became zero. It is almost never a genuine 1970 date — check what your code actually passed.

Seconds or milliseconds — how do I tell?

Count the digits. Ten digits is seconds and will be a date in the 2001–2286 range. Thirteen is milliseconds. JavaScript uses milliseconds; most Unix tooling, databases and APIs use seconds.

Sources

• POSIX.1-2017 — Seconds Since the Epoch