How it works

A useful distinction

Recheck recurring meetings by date

A meeting’s time difference can change when regions move their clocks on different dates. Convert each occurrence around seasonal transitions; copying a single converted time into a repeating invitation can shift attendance by an hour.

Choosing named zones or fixed UTC offsets

A named time zone can apply date-dependent offset rules, while a fixed UTC offset stays at the entered difference. Use a date when converting a meeting or event across locations, because seasonal clock changes can affect the result. Review ambiguous or skipped local times near a clock change. A fixed-offset conversion should not be mistaken for a complete regional time-zone calendar.

Time-zone conversion: formula and assumptions

Resolve the local wall-clock time to a UTC instant, then format that instant in the destination zone.

Nonexistent local times during a spring-forward transition are rejected. Repeated fall-back times require a chosen occurrence.

Fixed UTC offsets: formula and assumptions

Destination civil time = source civil time − source offset + destination offset.

Fixed offsets do not observe daylight-saving changes. Use named zones to apply a location’s timezone rules.

Worked example

Using the Time-zone conversion example:

Inputs

Date: 2026-10-09
Local time: 12:00
From time zone: America/New_York
To time zone: Europe/London
Repeated local time: Use earlier occurrence

Method

Resolve the local wall-clock time to a UTC instant, then format that instant in the destination zone.

Result

Example result

Destination local time: 2026-10-09 17:00:00

Destination time zone: Europe/London

UTC instant: 2026-10-09T16:00:00.000Z

Source offset minutes: -240

Destination offset minutes: 60

These are illustrative inputs. Change the values to explore a different scenario.

What to keep in mind

Timezone rules are supplied by the browser/runtime's IANA timezone data; future rule changes require updated software.

Formula references

References explain the underlying methods; they do not imply endorsement or individual professional review of Finflo.

Common questions

Why can a named-zone spring-forward time be rejected?

Some local times never occur when clocks jump forward. A repeated fall-back time instead requires choosing which occurrence is intended.

Does a fixed UTC offset follow daylight-saving changes?

No. It represents only the entered offset. Use a named IANA zone when location-specific transition rules are required.

Prepared by Finflo. See our methodology and editorial policy.

Time Zone Calculator

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