How it is calculated
A Unix timestamp is the number of seconds since 1 January 1970 00:00:00 UTC (the epoch), ignoring leap seconds.
date = new Date(timestamp × 1000) timestamp = floor(date ÷ 1000)13-digit values are usually milliseconds (as in JavaScript); 10-digit values are seconds.
Worked example
Timestamp 1800000000 is 15 January 2027, 08:00:00 UTC.
Frequently asked questions
What is the year-2038 problem?
Systems storing timestamps as 32-bit integers overflow on 19 January 2038; modern systems use 64-bit.
Does a timestamp depend on time zone?
No, it is one global instant; the time zone only affects how it is displayed.
How do I get it in PHP?
With time(); in JavaScript Math.floor(Date.now() / 1000).