# The hour that happens twice

English reference summary. Updated 2026-09-26. Reference facts checked 2026-09-07. Author: Oleksii Shevchenko, SHEV Software.
Canonical article: https://www.enerplain.com/notes/daylight-saving-time/

In the synthetic Europe/Warsaw example, 2026-10-25T00:15:00Z is 02:15 +02:00 locally; 2026-10-25T01:15:00Z is 02:15 +01:00. These are separate instants.

Preserve the instant, explicit offset, named time zone where needed, and the interval-start/end convention. Do not deduplicate on a bare local clock string. Do not invent a missing offset: obtain the source convention or a reliable disambiguator. Future time-zone rules can change.

References: [RFC 3339](https://www.rfc-editor.org/info/rfc3339/), [IANA time zones](https://www.iana.org/time-zones). This is not an OEM compatibility claim.