Find working-hour overlap across time zones
Compare two people’s local work hours on a chosen date. Enter named IANA time zones and a start and end for each shift. The calculator resolves the date’s actual offsets, including daylight saving changes, then shows any shared interval in both zones and UTC.
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Ready. Everything you enter in this tool stays local.
How to use it
- Set the local start date Choose the calendar date on which each person’s shift starts in their own time zone. This is the same written date for both people, not one shared midnight instant. The supported date range begins in 2000 and ends in 2100.
- Enter zones and hours Use a suggested IANA identifier or type another valid one, such as America/New_York or Asia/Kolkata. Enter local hours in 24-hour notation. An end earlier than its start means the shift ends on the following local calendar date.
- Calculate and check dates Select Calculate overlap. Read the full start and end dates, not only the clock hours. A valid overlap is shown in each person’s zone and UTC; disjoint shifts receive a clear no-overlap result.
Daylight saving is date specific
The calculation uses the browser’s time-zone data for the selected date. It does not reuse today’s offset or assume every local day lasts twenty-four hours. A shift across a spring or autumn clock change can therefore have a different elapsed duration from its wall-clock appearance.
A boundary inside a skipped hour does not identify a real instant. A boundary inside a repeated hour can identify two instants. This tool rejects both cases and asks you to choose another boundary instead of silently guessing which occurrence you meant.
Compare shifts rather than recurring calendars
This calculator compares two individual shifts. Overnight shifts start on the chosen local date and end on the next local date. Equal start and end times are rejected because they do not explain whether you meant no work or a full day.
It does not add holidays, breaks, weekends, employment rules, or recurring schedules. For a meeting near a clock change, recalculate using the actual meeting date and confirm the displayed dates with both people.
What this tool can and cannot do
- Accuracy depends on the time-zone database installed in the browser; future government rule changes may require a browser update.
- Time-zone abbreviations and numeric offsets are not substitutes for a named zone. UTC is accepted explicitly.
FAQ
Does an overnight shift work?
Why is an hour rejected on a DST date?
Can two shifts touch without overlapping?
Sources
- MDN: Date and time formatting · Checked 2026-10-28
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