Instants and wall-clock times
An instant is unique. A local wall-clock reading can be ambiguous because it combines calendar fields with regional rules. Converting from a timestamp to a zone is deterministic; converting a local date to a timestamp can require a choice.
Gaps and overlaps
When clocks move forward, some local times never occur. When clocks move backward, an interval occurs twice with different UTC offsets. Production scheduling interfaces should reject gaps and ask which offset to use for overlaps.
Store the right information
For a past event, store the instant. For a future recurring event, you may also need the IANA zone because its offset can change. A fixed offset cannot represent all regional rules.
Worked examples in New York
| Local wall time | What happens | Required action |
|---|---|---|
| 2024-03-10 02:30 | The spring jump skips this time | Choose an existing time |
| 2024-11-03 01:30 | The autumn change repeats this time | Choose UTC−04:00 or UTC−05:00 |
| 2024-11-03 03:00 | The time occurs once | Convert normally |
For the repeated 01:30 reading, the earlier occurrence is 2024-11-03T05:30:00Z; the later is 2024-11-03T06:30:00Z. Both are legitimate. An interface that silently chooses one can schedule an event an hour away from the user's intention. Test both choices in date to timestamp or the two-zone converter.
PST, PDT and a named time zone
PST is a fixed standard offset of UTC−08:00. Los Angeles uses PDT, UTC−07:00, during its daylight-saving period. Beijing does not currently change its UTC+08:00 offset seasonally. Consequently, Los Angeles is usually 16 hours behind Beijing in standard time and 15 hours behind during daylight saving.
On October 13, 2026 at 09:26 in Beijing, Los Angeles reads October 12 at 18:26. Selecting America/Los_Angeles applies the date's regional rule. Treating PST as the year-round local zone would give 17:26 instead.
Check a future appointment
Enter the actual appointment date, not today's offset. Choose named zones for both regions and check the destination date as well as its clock time. Keep the source zone with a recurring meeting definition. Rules can change through government decisions, so important future schedules should be checked with up-to-date runtime time-zone data.
A timestamp-to-zone conversion formats an already known instant; a two-zone conversion first interprets a source wall time. They answer different questions.