Elapsed time from the UTC epoch
Unix time counts from 1970-01-01 00:00:00 UTC. A value of zero is that exact instant.
The timestamp does not store a region; the same instant is formatted differently in UTC and local time.
Ten-digit and thirteen-digit values
Current second values are commonly around ten digits and millisecond values around thirteen. Misreading the unit produces an implausibly distant date.
Check automatic detection and prefer the documented unit of the source system.
- Seconds: seconds since epoch
- Milliseconds: seconds × 1,000
- 1 second = 1,000 milliseconds
Compare local time with UTC
Korea Standard Time is nine hours ahead of UTC. UTC 00:00 displays as 09:00 on the same date in Korea.
When comparing browser, database, and server logs, identify which systems store UTC and which store local values.
Daylight saving and ambiguous strings
Regions using daylight saving can change offset by date; one fixed offset may create a one-hour error.
A date string without zone information may be interpreted locally. Prefer ISO values with an explicit zone in APIs.