Unix Timestamp Converter

A Unix timestamp is a single integer counting seconds from 1970-01-01T00:00:00Z, the POSIX epoch. Its appeal is that it is monotonic, arithmetic-friendly, and free of any timezone or calendar assumptions: comparing two moments is an integer comparison, and a duration is a subtraction. A ten-digit value is conventionally seconds and a thirteen-digit value milliseconds, which is why this converter treats anything above 9,999,999,999 as milliseconds. Dates before 1970 are simply negative, so -1 is 1969-12-31T23:59:59Z. Formatting for humans is a separate step governed by ISO 8601 and its internet profile RFC 3339, which fixes the T separator and the Z or numeric offset suffix. Two subtleties are worth knowing. POSIX time is not a count of elapsed SI seconds: it is defined so that every day contains exactly 86,400 seconds, so an inserted leap second is absorbed by repeating or smearing a value rather than incrementing the counter. Twenty-seven leap seconds have been inserted since 1972, and the difference between TAI and UTC now stands at 37 seconds. The other is the 32-bit signed limit of 2,147,483,647, which is reached at 2038-01-19T03:14:07Z, after which a naive counter wraps to 1901. A practical trap when converting in the other direction is that JavaScript parses a bare date string as UTC but a date-and-time string with no offset as local time, so the same input can yield timestamps that differ by your UTC offset.

Unix time was a Bell Labs implementation detail before it was a standard. The First Edition Unix Programmer's Manual of November 1971 documented system time as a 32-bit count of sixtieths of a second since the start of 1971, ticking with the mains line frequency. At 60 ticks per second a 32-bit counter exhausts itself in roughly two and a half years, which was plainly unworkable, so the granularity was changed to one second and the epoch was moved back to the start of 1970 as the system was rewritten in C by Ken Thompson and Dennis Ritchie. A one-second tick pushed the overflow out to 2038 instead. The convention escaped Unix through standardization. The POSIX specification, first published as IEEE Std 1003.1 in 1988, fixed the epoch and the seconds-since-epoch definition of time_t, and effectively every network protocol, database, and log format since has either used it directly or converted to it. Migration away from 32-bit time_t took decades: OpenBSD switched to a 64-bit time_t on all architectures with version 5.5 in 2014, and the Linux kernel gained Y2038-safe interfaces for 32-bit platforms in version 5.6 in 2020. Embedded controllers and long-lived legacy databases remain the residual risk.