Question 1

What is a Unix timestamp?

Accepted Answer

A Unix timestamp is the number of seconds (or milliseconds in many programming languages) that have elapsed since the Unix epoch — midnight UTC on January 1, 1970. It is the most portable way to represent a specific moment in time, because it is a single integer with no timezone or locale ambiguity.

Question 2

How do I tell if a timestamp is in seconds or milliseconds?

Accepted Answer

A Unix timestamp in seconds for a date in the 2020s is a 10-digit number (around 1,700,000,000). A millisecond timestamp for the same period is a 13-digit number. This tool auto-detects: numbers with 11 or fewer digits are treated as seconds; anything longer is treated as milliseconds.

Question 3

What is the Y2K38 problem?

Accepted Answer

On 32-bit systems that store Unix time as a signed 32-bit integer, the maximum value is 2,147,483,647 — which corresponds to 03:14:07 UTC on 19 January 2038. After that moment, the counter overflows and the date resets to 1901 on systems that have not been upgraded. Modern 64-bit systems are not affected, since a 64-bit counter can represent dates hundreds of billions of years into the future.

Question 4

What is ISO 8601?

Accepted Answer

ISO 8601 is the international standard for representing dates and times. A full ISO 8601 datetime looks like 2026-08-18T14:30:00.000Z — year, month, day, a T separator, hours, minutes, seconds, optional milliseconds, and a Z suffix meaning UTC (or ±HH:MM for other timezones). It is unambiguous, locale-independent, and sorts correctly as a string — making it the preferred format for APIs, databases, and log files.

Question 5

Why does "local time" differ from UTC?

Accepted Answer

Local time is adjusted by your browser's system timezone offset. The Unix timestamp and the ISO UTC values are always the same absolute moment in time — local time is just the same moment expressed in your timezone. The offset shown in your local time string (e.g., GMT+0530 for IST) tells you how far your timezone is from UTC.

Event	Unix (seconds)	ISO 8601 (UTC)
Unix epoch (start)	0	1970-01-01T00:00:00Z
Y2K (Year 2000)	946684800	2000-01-01T00:00:00Z
Unix time 1 billion	1000000000	2001-09-09T01:46:40Z
Unix time 1.5 billion	1500000000	2017-07-14T02:40:00Z
Unix time 2 billion	2000000000	2033-05-18T03:33:20Z
Y2K38 (32-bit overflow)	2147483647	2038-01-19T03:14:07Z

Unix Timestamp & Date Converter

Notable Unix timestamps

Common questions