Original reference guides · Reviewed 8 October 2026
Wrong hour? Separate the instant from its display
Start here when: everything is shifted by an hour, or an event moves when you travel.
Three ICS spellings answer different questions: UTC uses a trailing Z; a TZID associates local clock time with a named zone; a floating time has neither. A TZID reference needs a corresponding VTIMEZONE definition for a self-contained RFC 5545 file. RFC 5545, date-time and zone rules.
Compare these original examples
- DTSTART:20270115T150000Z means a particular UTC instant.
- DTSTART;TZID=Europe/Paris:20270115T160000 expresses a Paris clock time, with the zone definition supplied elsewhere in the calendar.
- DTSTART:20270115T160000 expresses a floating 16:00, without a fixed worldwide instant.
The second line is an explanatory fragment, not a complete downloadable calendar. Do not copy a TZID line into a file and assume every importer will supply the omitted zone rules. The downloadable UTC example avoids that dependency for a single fictional event.
Diagnose before editing
- Find the source's intended meaning: a worldwide broadcast instant, a meeting at a venue, or a personal activity at the same local hour wherever you are.
- Inspect the actual DTSTART and DTEND. A title saying “Paris time” cannot replace structured time-zone information.
- Note the viewing calendar's zone. Compare the event in two displayed zones without changing its underlying time.
- For repeating events, examine dates on both sides of a seasonal clock change. One correct January occurrence does not demonstrate a correct summer schedule.
Google provides separate controls for calendar display zones and an individual event's zone. Its documentation also allows additional zones beside the calendar grid, useful for checking an apparent shift without rewriting the event. Google time-zone guide.
Apple Calendar for Mac exposes a Floating option after time-zone support is enabled. Apple describes it as keeping an event from moving when viewing another zone. That is useful for a local-clock routine, but an unsuitable repair for a meeting that everyone must attend simultaneously. Apple Mac time-zone guide.
The floating-time download provides a separate synthetic test for the third form.
Avoid: removing every Z until the screen looks right. That changes the meaning of the data and can hide the original error.