Question 1

What is a Unix timestamp (epoch time)?

Accepted Answer

It counts the seconds elapsed since 1970-01-01 00:00:00 UTC (the Unix epoch). It carries no time zone information, so one instant maps to a single timestamp worldwide.

Question 2

What is the difference between UTC, GMT and local time?

Accepted Answer

UTC is today's coordinated time standard; GMT is the older Greenwich mean solar time and is often used interchangeably with UTC in everyday use. Local time equals UTC plus the region's offset, which is why one timestamp shows different clock readings in different zones.

Question 3

What are time zones and UTC offsets?

Accepted Answer

A time zone is a region that keeps a common clock. Its offset is the difference from UTC, for example Beijing is UTC+08:00. At the same instant, UTC+08:00 is eight hours ahead of UTC.

Question 4

What is daylight saving time (DST)?

Accepted Answer

Some regions move their clocks forward by one hour in summer (for example UTC+01:00 to UTC+02:00) to make better use of daylight. During DST a zone's offset changes, so when converting timestamps you must check whether DST applied on that date, otherwise the result can be off by one hour.

Question 5

How do I tell second and millisecond timestamps apart?

Accepted Answer

Second timestamps usually have 10 digits (for example 1789210864); millisecond timestamps have 13 (for example 1789210864000) and equal the second value multiplied by 1000. JavaScript's Date.now() returns milliseconds. Mixing the two units shifts the time by a factor of 1000.

Question 6

What are ISO 8601 and RFC 3339?

Accepted Answer

Internationally recognised date-time formats, such as 2026-09-12T11:01:04Z, or with an offset such as 2026-09-12T19:01:04+08:00. A trailing Z means UTC. They are easy to exchange between systems and sort correctly as text.

Question 7

What are leap seconds, and do they affect timestamps?

Accepted Answer

To keep UTC in step with Earth's slightly slowing rotation, one second is occasionally inserted (for example 23:59:60). Unix timestamps assume exactly 86,400 seconds per day and do not record leap seconds, so rely on a standard date library when exact alignment matters.

Question 8

What is the year 2038 problem?

Accepted Answer

A signed 32-bit integer can only represent times up to 2038-01-19 03:14:07 UTC, after which it overflows. Modern systems use 64-bit integers, which last for about 292 billion years; this tool works in 64-bit.