How to Convert Time Zones: UTC Offsets, Day Rollover and DST

Everything is measured from UTC

Every time zone is defined as an offset from Coordinated Universal Time, written like UTC−5 or UTC+9. The offset tells you how many hours that zone runs ahead of or behind the global reference. New York in winter is UTC−5, meaning it is five hours behind; Tokyo is UTC+9, nine hours ahead. Once you know both offsets, converting between the two zones is pure arithmetic.

The rule of direction is simple: moving east adds hours, moving west subtracts them. To go from a western zone to an eastern one, add the gap; to go the other way, subtract it. The size of the gap is just the difference between the two offsets — from UTC−5 to UTC+9 the gap is 14 hours, because −5 to +9 spans 14.

A worked example with a day rollover

Convert 9:00 PM Monday in Los Angeles (UTC−8) to Tokyo (UTC+9). The gap is 17 hours, and Tokyo is east, so add 17 hours to 9:00 PM. Adding 3 hours reaches midnight, which rolls the date to Tuesday; the remaining 14 hours land at 2:00 PM. The answer is 2:00 PM Tuesday in Tokyo — the same instant, but a full day and afternoon later on Tokyo's clock.

The day rollover is the step people forget. Whenever adding hours pushes the time past midnight, the date moves forward; whenever subtracting hours drops it before midnight, the date moves back. For big east-west gaps like this one, being a day off is easy, which is exactly why a converter that tracks the date alongside the time is worth using.

The daylight saving caveat

Offsets are not fixed year-round. Many places shift their clocks for daylight saving, so New York is UTC−5 in winter but UTC−4 in summer, and London is UTC+0 in winter but UTC+1 in summer. That means the gap between two cities can change by an hour depending on the date — and the two ends do not always switch on the same day, so for a few weeks each year the usual gap is temporarily off by an hour.

The practical fix is to always use the offset that applies on the exact date you are converting, not the one you happen to remember. This is where a converter earns its keep: it stores each zone's daylight-saving rules and picks the right offset for the chosen date automatically, so you never have to track which cities are currently on summer time.

Frequently asked questions

Which direction adds hours and which subtracts?

Moving east adds hours; moving west subtracts them. A city with a higher UTC offset is further east and ahead in time, so converting toward it means adding the gap. A city with a lower offset is west and behind, so converting toward it means subtracting. If your result is negative, add 24 and step the date back a day.

How do I handle the day changing?

Whenever your addition pushes the clock past midnight, advance the date by one day; whenever a subtraction drops it before midnight, move the date back a day. For large offsets — say converting between Los Angeles and Tokyo — the calendar date almost always changes, so track the date deliberately rather than only the clock time.

Do I need to worry about daylight saving time?

Yes, when either zone observes it. A city's offset can shift by one hour in summer, so the gap between two places is not constant across the year. Because the two zones may not switch on the same date, the gap is briefly unusual each spring and autumn. Always use the offset in force on the specific date, which a converter handles automatically.

What about half-hour and 45-minute zones?

Some zones are not whole hours: India is UTC+5:30 and Nepal is UTC+5:45. The method is identical — you just add or subtract minutes as well as hours. Converting into India from UTC means adding 5 hours 30 minutes, so 12:00 noon UTC becomes 5:30 PM. A converter handles these fractional offsets without you doing the minute math.