4 min read · Time
Computers usually store time as a single number: the count of seconds since a fixed starting moment called the Unix epoch. It is simple, compact, and the same everywhere on Earth.
What a Unix timestamp is
A Unix timestamp is the number of seconds that have passed since 00:00:00 UTC on January 1, 1970. For example, 1,800,000,000 is January 15, 2027 at 08:00:00 UTC.
Because it is always measured in UTC, a timestamp has no time zone. The same number means the same instant in Tokyo and in New York; only the way you display it changes.
Seconds or milliseconds?
Unix systems and most APIs use seconds, which today is a 10-digit number. JavaScript’s Date.now() and many databases use milliseconds, a 13-digit number. If a converted date lands in 1970 or thousands of years in the future, you probably mixed the two up.
Leap seconds and negative timestamps
Unix time ignores leap seconds: every day is treated as exactly 86,400 seconds. Dates before 1970 are simply negative numbers.
The year 2038 problem
Older systems store the timestamp as a signed 32-bit integer, whose largest value is 2,147,483,647. That moment is 03:14:07 UTC on January 19, 2038. One second later, those systems overflow to a negative number and read the date as December 1901. Modern systems use 64-bit timestamps, which will not run out for billions of years, but embedded devices and old file formats are still being updated.
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