Time zone overlap: finding the hours you share
Overlap is a number you can compute, and once you have it the scheduling argument usually ends — because most of those arguments are really about a constraint nobody had measured.
The short answer
Overlap is the intersection of two working days once both are expressed in the same reference. Two nine-to-five days n hours apart overlap for 8 − n hours, and not at all once n reaches eight.
So a 3-hour gap leaves 5 hours, a 5-hour gap leaves 3, and a 9-hour gap leaves nothing unless somebody moves their hours.
The formula, and where it stops working
For two standard eight-hour working days separated by n hours:
| Gap | Overlap | Feels like |
|---|---|---|
| 1–2 h | 6–7 h | Effectively same-office |
| 3 h | 5 h | Comfortable; US coast to coast |
| 5 h | 3 h | Workable; transatlantic |
| 7 h | 1 h | Tight; needs deliberate design |
| 8 h+ | none | Somebody must shift their hours |
The formula assumes rigid nine-to-five days on both sides. Real overlap is usually a little better, because people flex — the transatlantic pattern of a late-starting London and an early-starting New York is exactly this.
It also assumes the gap is stable. It often is not: if one side observes daylight saving and the other does not, your overlap changes twice a year without warning.
Widen it before you shrink the problem
Before concluding that a gap is unworkable, check three things.
Are the working days actually rigid? An hour of flex on each side turns a one-hour overlap into three. That is frequently the entire fix, and it costs nothing but agreement.
Is the overlap in the same place every day? A window that moves around is worse than a narrower one that does not, because nobody can plan around it.
Are you spending the window on the right things? The most common waste is using a two-hour overlap for a status meeting. Status is asynchronous by nature; unblocking, judgement calls and disagreement are not.
When the overlap is genuinely small
Some pairs cannot be fixed by flexing. London ↔ Sydney shares about 5 hours and both ends of it are unsociable.
Those setups work when they are designed for rather than endured:
- Write decisions down, including the reasoning. If a question can only be resolved in the overlap, the answer arrives a day late.
- Push decision authority outward. Someone who has to ask permission is idle for most of their day.
- Name one person on each side who is genuinely responsible for answering during the window.
- Use a handover document, not a handover meeting, wherever you can.
Overlap is a hiring input, not a discovery
The most expensive version of this is finding out after someone has started. Overlap is knowable during a hiring process in about a minute, and it changes whether a role works — see the scenario.
Recompute it twice a year
Any overlap involving two regions with different daylight-saving rules moves. The dates worth checking are mid-March and late October to early November, when the US and the EU are briefly out of step, and any time a country changes its rules — several have in recent years.
Related: When are two cities both awake? · Working across time zones
Timezone: World Clock Widget
A world clock, converter and live day/night map for iPhone — 325 cities, offline, no account.
Figures on this page are computed from the IANA time zone database. Last generated 2026-08-18.