Reading a Business Hours Overlap Matrix Across Time Zones

A city-pair page on this site answers a narrower question than “what is the time difference.” For each hour from 9:00 AM through 6:00 PM in one city, it shows the other city’s local time. A row is marked green when both clocks are inside 9:00 AM through 5:00 PM. The 6:00 PM row is on the table, and it is not marked green. The green rows are a planning default. A factory shift or a Ramadan timetable will not match them.

What a green row is

Take London and San Francisco. For most of the year San Francisco is eight hours behind. London’s 9:00 AM is 1:00 AM in San Francisco. The only green row is 5:00 PM in London, which is 9:00 AM in San Francisco. One hour, on the edge of both days.

People still book 8:00 AM San Francisco, which is 4:00 PM in London. London is inside a normal day. San Francisco is an hour before the flagged window, so that row stays unmarked. Book it if the California side has agreed to start early. 6:00 PM in London is 10:00 AM in San Francisco, and 6:00 PM is the row the grid leaves unmarked on every page.

In the mismatch weeks the gap is seven hours and the band grows. 9:00 AM in San Francisco is 4:00 PM in London instead of 5:00 PM. Those weeks run from the second Sunday of March to the last Sunday of March, and again from the last Sunday of October to the first Sunday of November. They are the kinder weeks on this route. They are not the new normal. When the eight-hour gap comes back, the green band shrinks again, and a recurring invite anchored in one city moves for the other.

New York’s morning is London’s afternoon

London and New York are five hours apart for most of the year, and four hours in those same mismatch weeks. At five hours, 9:00 AM in New York is 2:00 PM in London, and noon in New York is 5:00 PM in London. The green rows are that block: a New York morning against a London afternoon. 1:00 PM in New York is 6:00 PM in London, past the flagged window.

London’s morning is not in the overlap. By the time London sits down to lunch, New York has not started. Read the grid if a cheat-sheet put this meeting in the London late morning.

Hong Kong, Mumbai, and Sydney are three different gaps

Hong Kong and New York are twelve hours apart from the second Sunday of March to the first Sunday of November, and thirteen hours otherwise. Hong Kong’s 9:00 AM is 9:00 PM the previous evening in New York. There is no green row inside two ordinary office days. Any live call is someone’s evening. The fair version is a named rotation, or no live call. The remote team guide is the rotation. The handoff note is the other option.

India is a different distance. Mumbai and New York sit 9 hours 30 minutes apart from the second Sunday of March to the first Sunday of November, and 10 hours 30 minutes apart the rest of the year. India is UTC+5:30, and it does not change for daylight saving. A 9:00 AM call in New York is 6:30 PM in Mumbai during the shorter gap and 7:30 PM during the longer one. Both are after the flagged window, so this pair has no green row either. Rounding India to a whole hour makes you thirty minutes early forever. Scheduling with India keeps the :30 in the examples.

Sydney against the US East Coast is the other direction people mis-file as “about twelve.” New York and Sydney runs at 14, 15, or 16 hours depending on the season. The date changes under you. Read that page’s schedule before you save a series.

Pairs the grid will mostly paint green

Sydney and Tokyo are two hours apart while Sydney is on daylight time, from the first Sunday of October to the first Sunday of April, and one hour apart the rest of the year. Tokyo does not change. Almost the whole workday overlaps in both settings. The hour to refuse is the late one: 6:00 PM in Tokyo is 8:00 PM in Sydney during the two-hour setting.

Singapore and Sydney are three hours and then two, on Sydney’s dates. Singapore’s morning is a normal Sydney late morning. Singapore’s 5:00 PM is 8:00 PM or 7:00 PM in Sydney, which is when a Sydney office has thinned out. Keep the live call in Singapore’s morning and let it drift by an hour twice a year.

Three cities, and the meeting you save once

Add Singapore to London and San Francisco and a column where all three are inside 9:00 to 18:00 does not exist. The workable design is two-way calls plus a written note, which is what the San Francisco, Berlin, and Singapore week spells out with the hours. Forcing one invite for all three means the meeting sits on someone’s night every week, or it gets moved so often that people stop trusting the calendar.

A one-off is easy to check on the grid. A weekly series is the one that rots. The event has a time zone. That zone keeps its hour, and the other city moves when the clocks change. Which time zone should own the invite is how to pick the zone on purpose. Check the series on the Monday after each switch, not on a vague memory that “daylight saving happened.”

The grid will not know a Paris lunch, a Dubai afternoon in Ramadan, or a Sydney office in the first half of January. Those are staffing facts. The green row only means both clocks say the hour is between 9 and 6.