How to Calculate Time Difference Between Two Countries
The arithmetic is the easy part. Subtract one UTC offset from another and you have the time difference. New York is UTC-5, London is UTC+0, so London is 5 hours ahead.
The hard part is knowing which offset is in force on the day you care about. New York is UTC-5 only in winter and UTC-4 from March to November. London is UTC+0 only in winter and UTC+1 in summer. And the two switch on different dates, so for a few weeks each spring and one week each autumn the gap is 4 hours, and anyone relying on a remembered "5 hours" is wrong.
Here's the method that gets it right every time, and the cases where people most often slip.
Three Steps
1. Find both offsets for the date. Not the standard offset, the one in effect on that date. A city that observes daylight saving has two.
2. Subtract. If city A is UTC+X and city B is UTC+Y, then B minus A is the difference. A positive result means B is ahead of A.
3. Check it with a real time. The sum is only the first half. Convert one time in each direction and see if the answer makes sense.
Worked example, New York in January to Tokyo. New York is UTC-5 and Tokyo is UTC+9, so 9 minus (-5) is 14: Tokyo is 14 hours ahead. Check: 9:00 AM in New York is 11:00 PM the same day in Tokyo.
Another: Mumbai to London in January. London is UTC+0 and Mumbai is UTC+5:30, so London is 5 hours 30 minutes behind Mumbai. Check: 9:00 AM in Mumbai is 3:30 AM in London. The half hour is the part people drop when they do it in their heads.
The Gaps People Look Up
Calculated from the time zone database for a normal January and July (the second city is ahead of the first by this much):
| From | To | January | July |
|---|---|---|---|
| New York | London | 5 hours | 5 hours |
| New York | Mumbai | 10.5 hours | 9.5 hours |
| New York | Tokyo | 14 hours | 13 hours |
| New York | Sydney | 16 hours | 14 hours |
| London | Mumbai | 5.5 hours | 4.5 hours |
| London | Dubai | 4 hours | 3 hours |
| Berlin | Singapore | 7 hours | 6 hours |
| Los Angeles | New York | 3 hours | 3 hours |
Los Angeles and New York stay 3 hours apart all year because both change clocks on the same dates. The same goes for any two US cities, and for any two cities on the same side of the clock-change rules. When one city changes and the other doesn't, as with India, Japan, Dubai and Singapore, the gap moves by an hour twice a year.
Where the Gap Moves
One side changes, the other doesn't. New York and India are 10.5 hours apart in winter. When the US moves to daylight time in March, New York gets an hour closer to Mumbai: 9.5 hours. It goes back in November.
Both change, on different dates. In 2026 the US moved on March 8 and the UK on March 29, so for three weeks the New York to London gap was 4 hours. In 2027 it's two weeks, because the US change falls on March 14. In autumn the UK goes back on the last Sunday in October and the US on the first Sunday in November, so there's one week of 4 hours again. The exact dates are on the daylight saving page for the year.
The hemispheres are opposite. Sydney's summer is in December. New York and Sydney are 16 hours apart in January, when New York is on standard time and Sydney on summer time, and 14 hours apart in July. In between, for a few weeks around each change, it's 15. For example, 9:00 AM in New York on March 20, 2026 was midnight on Saturday in Sydney (15 hours). New York and Sydney are never both on standard time at the same time.
Why the Offsets Aren't Tidy
The idea of one-hour zones comes from the railways. In the US, the railroads adopted five standard time zones in November 1883. The International Meridian Conference in Washington in 1884 then agreed on Greenwich as the prime meridian for the world. Since then each country has fitted its own time to its own needs.
That's why India, which spans about 29 degrees of longitude, uses a single time, UTC+5:30, and why Nepal is on UTC+5:45. Governments choose their offsets, and they change them. That is also why a table you memorised can go out of date: British Columbia, for example, stopped changing its clocks after March 8, 2026.
What to Do Next
For a one-off conversion, use the time zone converter: enter a time, add the zones and set the date, and it applies the offsets in force on that date. For a pair you handle every week, the dedicated pages have a 24-hour table for both seasons, such as EST to IST and GMT to IST, and the time difference pages cover popular city pairs.
Related Articles
- EST to IST Converter: the 24-hour table for both seasons and the best hours for US and India teams.
- Daylight Saving Time 2026: When Does It Start and End?: the dates for the US, UK, EU and Australia, including the weeks that shift time differences.
- US Time Zones Explained: EST, CST, MST, PST and How They Work: which states use which zone, what the abbreviations mean, and the edge cases.