Number versus formatted date
A Unix timestamp is compact and convenient for arithmetic. ISO 8601 is readable and can state an offset. 2023-11-14T22:13:20Z uses Z to declare UTC and maps to Unix timestamp 1700000000 seconds.
Preserve the instant
2023-11-14T22:13:20Z and 2023-11-15T06:13:20+08:00 describe the same instant. Converting both to Unix time produces the same value. An ISO string without Z or an offset needs an external rule defining its time zone.
Round-trip choices
Use an ISO string with an explicit offset for interchange and logs meant for humans. Use a Unix value when the unit is clearly documented and compact arithmetic matters. Keep fractional seconds if the source contains meaningful precision.
Compare an offset date with Unix time
These date strings identify the same instant:
| Representation | Value |
|---|---|
| UTC | 2023-11-14T22:13:20Z |
| Beijing offset | 2023-11-15T06:13:20+08:00 |
| New York offset | 2023-11-14T17:13:20-05:00 |
| Unix seconds | 1700000000 |
The letter Z means a UTC offset of zero. A signed offset states how far the displayed clock is ahead of or behind UTC. Removing Z or the offset leaves a wall-clock reading that needs an interpretation zone. Adding Z to a Beijing clock reading does not convert it to UTC; it declares a different instant.
Perform a reliable round trip
Start with the timestamp converter, select seconds and enter 1700000000. Copy its ISO result. To verify the reverse direction, enter November 14, 2023 at 22:13:20 in date to timestamp, select UTC, and request seconds. The result should be the original number.
When communicating dates between systems, specify whether fractional seconds are supported. 2023-11-14T22:13:20.123Z maps to 1700000000123 milliseconds. A seconds-only destination cannot retain the 123-millisecond fraction.
Choose the format for the destination
ISO 8601 covers more forms than many parsers accept. RFC 3339 is a narrower internet timestamp profile; an extended calendar date, time and explicit offset is a practical interchange choice. A zone suffix such as [America/New_York] carries an IANA identifier but is not accepted by every ISO or RFC 3339 parser. Use the UTC ISO output when a receiver expects a plain machine timestamp, and retain the zone separately when regional scheduling rules matter.
Avoid locale strings such as 03/04/2026 in APIs: the date order is ambiguous. See time zones and DST before interpreting dates without offsets.