World Clock Time Converter
Instantly convert any time from one city to another, factoring in UTC offsets and daylight saving time. Use it when booking international flights, scheduling calls, or watching live global events.
About this calculator
Time zone conversion works by calculating the net offset between the source and target zones and adding it to the source time. The formula used here is: target_time = ((source_hour + source_minute / 60 + (target_offset − source_offset) + DST_adjustment) mod 24 + 24) mod 24. The source_minute / 60 term converts minutes to a fractional hour so the entire calculation stays in decimal hours. The DST_adjustment adds 1 hour when daylight saving is active in the target zone. Two nested mod-24 operations guarantee the result stays in the 0–24 range regardless of whether the raw sum is negative or exceeds 24. The decimal output represents hours past midnight — for example, 14.50 means 14:30 local time.
How to use
Convert 08:45 New York (UTC−5) to London (UTC+0) during British Summer Time (DST active). Step 1 — source decimal: 8 + 45/60 = 8.75. Step 2 — offset difference: 0 − (−5) = +5. Step 3 — DST: +1 (BST is active). Step 4 — raw sum: 8.75 + 5 + 1 = 14.75. Step 5 — apply mod 24: 14.75 mod 24 = 14.75, which equals 14:45 London time. So an 8:45 AM New York meeting corresponds to 2:45 PM in London during summer.
Frequently asked questions
How does daylight saving time affect world clock conversions?
Daylight saving time (DST) shifts clocks forward by one hour in participating regions during summer months. In this calculator, enabling the DST toggle adds 1 hour to the target time, reflecting that the target location is currently observing summer time. Because not all countries observe DST — and those that do change on different dates — always verify whether DST is active in both the source and target location on the specific date of your event.
What does a negative UTC offset mean in time zone conversion?
A negative UTC offset means the location is west of the prime meridian and behind UTC. For example, UTC−5 (Eastern Standard Time) is 5 hours behind London. When you subtract a negative offset — e.g., target (UTC+1) minus source (UTC−5) — you get +6, meaning the target location is 6 hours ahead. The calculator handles this automatically through signed arithmetic, so you just enter the correct offset with its sign.
Why does the calculator use two mod-24 operations instead of one?
A single mod 24 can return a negative number in JavaScript when the input is negative (e.g., −2 mod 24 = −2 in some engines). Adding 24 before the second mod ensures the result is always non-negative: (x mod 24 + 24) mod 24 maps any value — however negative — into the 0 to 23.99 range. This double-modulo pattern is a defensive programming convention for clock arithmetic.