Unix Timestamps: The Universal Language of Time
A Unix timestamp (also called Epoch time) is the number of seconds — or, in JavaScript, milliseconds — that have elapsed since midnight UTC on January 1, 1970, excluding leap seconds. Because it is a single integer independent of timezones, calendars, and formatting conventions, it is the standard way computers store and compare moments in time. A Unix timestamp converter turns that opaque number into a readable date and vice versa.
Seconds vs. Milliseconds
Timestamps appear in two common flavors: 10-digit seconds (e.g., 1710000000) and 13-digit milliseconds(e.g., 1710000000000). JavaScript's Date object counts in milliseconds, so when converting from a seconds-based API you must multiply by 1000. This tool auto-detects which form you supplied by inspecting the digit count.
The Year 2038 Problem
Systems that store time in a signed 32-bit integer (time_t) top out at 2,147,483,647 seconds — 03:14:07 UTC on January 19, 2038 — after which they overflow into negative numbers. This is the infamous Year 2038 problem. Modern 64-bit systems, which most software runs on today, are unaffected and can represent dates hundreds of billions of years into the future.
Converting and Formatting
To convert a timestamp to a date, use new Date(timestamp * 1000). From there, the Date object exposestoISOString() for an unambiguous UTC string, toUTCString() for a human-readable GMT value, andtoLocaleString() for a timezone-aware rendering. Going the other way — date to timestamp — you readDate.getTime() for milliseconds or Math.floor(Date.getTime() / 1000) for seconds.
Timestamps and the Future
Timestamps comfortably represent both the past and the future. Future values power scheduling, cache expiration, JWT exp claims, and countdown timers. Because they are monotonic and timezone-free, they are ideal for measuring elapsed durations and ordering events across distributed systems.