You add or subtract a whole number of hours, check the clock and book the call. Then someone joins 30 minutes late, or you do. Time zones are not always whole hours apart, and that single detail is behind a lot of missed meetings. Here is the simple method that gets it right every time.
What a time zone actually is
A time zone is an area that follows one uniform standard time for legal, commercial and social purposes. Every zone is described by its offset from UTC, the world’s reference clock. Offsets run from UTC−12:00 at one end to UTC+14:00 at the other, which means two places on Earth can be up to 26 hours apart on the clock.
The common belief is that each zone is one full hour away from its neighbour. Most are, but not all. India uses UTC+05:30, a half-hour offset, and Nepal uses UTC+05:45, a quarter-hour offset. If you round those to 5 or 6 hours, every answer you calculate will be wrong by 15 or 30 minutes.
The time difference formula
You only need two steps, and both work with any offset, whole hour or not.
Time difference = UTC offset of place B − UTC offset of place A
If the result is positive, place B is ahead of place A. If it is negative, place B is behind. Then:
Local time in B = local time in A + the time difference
In plain words: find both offsets, subtract the one for where you are from the one for where you are calling, and add that gap to your own clock. Keep the minutes in the calculation instead of rounding them away.
Three worked examples from India
India to Nepal
Difference = +5:45 − (+5:30) = +0:15. Nepal is just 15 minutes ahead. When it is 10:00 AM in New Delhi, it is 10:15 AM in Kathmandu. Round both countries to the nearest hour and you would wrongly conclude they share the same time.
India to New York in summer
New York is on UTC−4 during daylight saving time. Difference = −4:00 − (+5:30) = −9:30. New York is 9 hours 30 minutes behind, so 6:00 PM in Mumbai is 8:30 AM in New York, a sensible slot for an evening call with someone starting their workday.
India to California, summer vs winter
California is on UTC−08:00 in standard time and switches to UTC−07:00 during daylight saving time. India stays on +5:30 all year, so the gap changes with the season:
- Summer: −7:00 − (+5:30) = −12:30. At 9:00 PM on Monday in Bengaluru, it is 8:30 AM on Monday in California.
- Winter: −8:00 − (+5:30) = −13:30. The same 9:00 PM in Bengaluru is now 7:30 AM in California.
Large gaps like these often cross midnight. At 10:00 AM on Tuesday in India, California in summer is 10:00 − 12:30, which goes below zero: add 24 hours and step back one day, and you get 9:30 PM on Monday.
The mistakes that throw the answer off
- Rounding to whole hours. India (+5:30) and Nepal (+5:45) are the classic traps. Write the minutes down and carry them through.
- Forgetting daylight saving time. The same pair of cities can give a different answer depending on the date. Most places that observe it move forward one hour, but Lord Howe Island in Australia moves forward only 30 minutes. Always use the offset that is active on the day you care about.
- Assuming zones follow longitude. Time zones follow the borders of countries and their regions rather than strict lines on the globe, because places that communicate a lot find it practical to share one clock. Two cities at a similar longitude can sit in different zones.
- Subtracting in the wrong order. It is always “where you are calling” minus “where you are”. Flip it and the sign flips with it, turning a 9:30 delay into a 9:30 lead.
A quick routine before you book anything
Look up both current UTC offsets, including minutes and any daylight saving shift on that date. Subtract yours from theirs. Add the result to your clock, and if you land below 0:00 or above 24:00, adjust the day. It takes under a minute and saves the awkward half-hour wait on an empty video call.
