How the time difference is calculated
Each city has a fixed offset from UTC (Coordinated Universal Time). Converting a time between two cities just means adding the difference between their offsets, then carrying any overflow into the date.
- baseHour/baseMinute — the time you enter in the origin city
- toOffset − fromOffset — difference between the two cities' UTC offsets
- mod 1440 — 1440 minutes = 24 hours, wraps the time of day
- floor(result/1440) — how many days the result shifts, negative or positive
More detail
Why the date can shift
Adding or subtracting hours can push the result past midnight or before 00:00. When that happens the calculator carries the overflow into the Date shift output — e.g. 9:00 AM in Seoul is 7:00 PM the previous day in New York, a 14-hour gap that crosses the date line.
Standard time, not daylight saving
This calculator uses each city's fixed standard-time offset (e.g. UTC−5 for New York, UTC+1 for Paris). During daylight saving months many cities set clocks forward by 1 hour, which briefly changes the real gap by 1 hour — check the destination's local DST status for exact scheduling.
Practical tip. For scheduling international calls, pick a base hour inside your own business hours and check the Hour difference output — a large negative number usually means the other city is still asleep.
Frequently asked questions
What time is it in New York when it's 9:00 AM in Seoul?
With the default inputs above, 9:00 AM in Seoul (UTC+9) converts to 7:00 PM the previous day in New York (UTC−5) — a 14-hour difference that also shifts the date back one day.
Does this calculator account for daylight saving time?
No — it uses fixed standard-time UTC offsets only (per the disclaimer above). If only one of the two cities is observing daylight saving time, the real gap is 1 hour different from what's shown; if both are (London and Paris in July), it is not.
How does a half-hour offset like Delhi (UTC+5:30) work?
Each city's offset is taken in hours — Delhi is 5.5, not 5 — and the formula turns the difference into minutes with (toOffset − fromOffset) × 60. From Seoul that is (5.5 − 9) × 60 = −210 minutes, so a half-hour zone needs no special case.
Why does the Date shift output sometimes show a different day?
Adding the hour difference can push the result past midnight (00:00) or before it. The calculator carries that overflow with floor(result / 1440) into the Date shift output, so you know whether to say "today", "yesterday", or "tomorrow" when scheduling.