The Best Time to Call Internationally — Overlap Windows for Major City Pairs
Every pair of cities has a small set of hours when a call is comfortable for both sides — and for some pairs, no such hours exist at all. Here are the practical windows for the pairs people actually call between, and a simple method to work out any pair yourself.
The idea: overlap windows
Assume a standard working day of roughly 9:00–18:00 on each side. The usable call window is the intersection of those two ranges once you shift one of them by the time difference. Three cases come up:
- Comfortable overlap (0–6 hours apart). Several shared working hours. Example: London–Dubai.
- Narrow overlap (6–10 hours apart). One side's early morning meets the other's late afternoon. Example: London–Seoul.
- No overlap (11+ hours apart). Someone has to take the call outside working hours. Example: New York–Seoul. The only fair fix is rotation — more on that below.
Quick reference: common city pairs
Windows below are given in each city's own local time, using the period when most of the year applies. Differences marked with a range change with daylight saving time, so always confirm the live difference on the world clock before you send an invite.
| Pair | Time difference | Good window | Notes |
|---|---|---|---|
| New York ↔ London | 5 h (briefly 4 h in spring) | NY 9:00–13:00 = London 14:00–18:00 | The classic transatlantic window; mornings in NY are golden. |
| San Francisco ↔ London | 8 h | SF 8:00–10:00 = London 16:00–18:00 | Narrow; SF early morning only. |
| New York ↔ São Paulo | 1–2 h | Almost the whole day | Brazil no longer observes DST, so the gap moves with US DST only. |
| London ↔ Dubai | 3 h (4 h in UK winter) | London 9:00–15:00 = Dubai 12:00–18:00 | UAE works Monday–Friday since 2022; weekends align. |
| New York ↔ Mumbai | 9.5 h (10.5 h in US winter) | NY 8:30–9:30 = Mumbai 18:00–19:00 | India's +5:30 offset never changes; the US side does. |
| London ↔ Seoul | 8 h (9 h in UK winter) | London 8:00–10:00 = Seoul 16:00–19:00 | Korea has no DST; UK summer narrows the gap. |
| New York ↔ Seoul / Tokyo | 13 h (14 h in US winter) | NY 18:00–20:00 = Seoul 7:00–10:00 (next day) | No working-hour overlap exists. NY evening ↔ Seoul morning, or NY 7:00–8:00 ↔ Seoul 20:00–22:00. |
| London ↔ Sydney | 9–11 h (both ends shift, in opposite seasons) | London 7:00–9:00 = Sydney 17:00–19:00 | The difference changes four times a year; never memorize it. |
| San Francisco ↔ Singapore | 15 h (16 h in US winter) | SF 17:00–19:00 = Singapore 8:00–10:00 (next day) | Effectively "yesterday evening meets tomorrow morning." |
How to find the window for any pair
- Get today's actual difference. Don't compute it from remembered offsets — DST changes them. Pin both cities on the gworldtime globe and read the live difference badge.
- Shift one working day. If B is +8 from A, then A's 9:00–18:00 is B's 17:00–2:00. The intersection with B's own 9:00–18:00 is 17:00–18:00 — a one-hour window.
- Widen the day honestly. Many people accept 8:00 starts or 19:00 ends. Two "stretch hours" often turn a zero-overlap pair into a workable one — but say explicitly that you know it's outside normal hours.
- Check the date, not just the hour. Across the Pacific, "Tuesday 18:00" in California is "Wednesday morning" in Asia. A missing day label is the single most common cause of missed calls.
The DST ambush weeks
Because regions change their clocks on different dates, pairwise differences temporarily shift twice a year:
- Mid-to-late March: the US springs forward two to three weeks before Europe, so New York–London is 4 hours instead of 5 for a stretch.
- Late October–early November: Europe falls back about a week before the US, briefly making the same pair 4 hours again.
- Southern hemisphere pairs (Sydney, Auckland, Santiago) shift in the opposite season, which is why London–Sydney changes four times a year.
A recurring weekly call that was fine in February can silently land an hour off in March. See our 2026–2027 DST calendar for the exact dates to watch.
Being fair when there is no good hour
For pairs like New York–Seoul, someone is always inconvenienced. Teams that handle this well do three things:
- Rotate the pain. Alternate who takes the early/late slot meeting by meeting, or month by month.
- Shrink the synchronous part. Send the material in advance; use the call only for decisions. A 25-minute call at 7:00 is far easier to accept than a 60-minute one.
- Write times unambiguously. State the city with every time ("Thu 18:00 New York / Fri 08:00 Seoul") or send a calendar invite, which converts automatically. Avoid bare abbreviations like "CST", which means three different offsets on three continents.
See both clocks at once
The fastest way to sanity-check a proposed time is to look at both cities side by side. On the gworldtime 3D globe you can pin up to five cities and read their live local times and pairwise differences — including whether it's already tomorrow on the other side.
Read next: Scheduling meetings across time zones · DST dates for 2026–2027 · How time zones work