Date and Time Calculator
Compare two local date-times across the same or different time zones, with daylight-saving gaps, repeated clock times and the difference between elapsed and wall-clock time made explicit.
True elapsed time is calculated after both local times are resolved to instants. No net clock change between the selected instants.
Context matters.
- Time-zone conversion uses the browser's implementation of the IANA time-zone database through Intl.DateTimeFormat.
- Time-zone laws can change. Recalculate future events after the device or browser receives time-zone data updates.
Sources & scope
The calculation guide and this source trail show what the result means, where the rule came from and when it was last reviewed. This is source-and-calculation QA, not a claim of medical, legal, tax or other professional review; time-sensitive decisions still belong with the linked authority.
Resolve local clocks before subtracting instants.
A local date and time is not always one instant: spring-forward can create a time that never occurs, while fall-back can repeat one. CalcSpan validates that mapping before calculating duration.
- 01Enter each local date-time
Choose the date, clock time and IANA city-based time zone for both endpoints.
- 02Resolve DST ambiguity
If a fall-back repeats the local time, choose whether the earlier or later occurrence should be used.
- 03Compare two answers
True elapsed time comes from instants; wall-clock difference compares the typed local readings. The offset effect explains any gap.
A local time skipped during spring-forward is rejected rather than silently moved forward or backward.
When fall-back creates two instants with the same local clock reading, the result labels the ambiguity and uses the selected occurrence.
The calculation uses the browser's Intl implementation of the IANA time-zone database and therefore benefits from browser or operating-system updates.
Rule set 2.0.0. Last reviewed 2026-08-22. Next review due 2027-02-22.
What to know before using the result.
Why can two hours on the clock equal one elapsed hour?
During a spring-forward transition, the local clock skips an hour. CalcSpan resolves both inputs to real instants and shows the true elapsed duration beside the wall-clock difference.
What is an ambiguous local time?
During fall-back, a clock time such as 1:30 AM can occur twice with different UTC offsets. Choose the earlier or later occurrence so the calculation maps to one instant.
What happens if a local time does not exist?
The tool stops and asks for a valid time before or after the DST gap. It does not guess.
Where do the time-zone rules come from?
The browser applies IANA time-zone data through the ECMAScript Intl.DateTimeFormat API. Political time-zone rules can change, so future events should be recalculated after updates.