How does the calculator find the UTC offset for each zone?

It uses the browser's built-in Intl API (specifically Intl.DateTimeFormat with the "shortOffset" timeZoneName option). The browser ships with a full IANA time-zone database, so daylight saving transitions are handled automatically and correctly — no external data is needed.

Does it handle daylight saving time automatically?

Yes. When you enter a custom date the offsets are computed for that exact instant, so a zone that is UTC+1 in winter and UTC+2 in summer will show the correct value whichever date you pick. Just make sure to enter a date that falls inside the relevant DST window.

What does the "diff from first" column mean?

It shows the signed hour difference between the first row (the reference zone) and each subsequent zone. A value of +5h 30m means that zone is five and a half hours ahead; -8h means it is eight hours behind. This makes it easy to answer "if it is 9 am in London, what time is it in Mumbai?" without mental arithmetic.

How do I plan a global meeting?

Switch to Custom date/time, type the proposed start time (for example 14:00) and pick the city or zone that time belongs to. Every other row instantly shows what 14:00 in that zone translates to locally. Look for an overlap where all participants are between 08:00 and 20:00.

Why might the diagram look compressed when zones are close together?

The horizontal scale spans from the lowest to the highest UTC offset in your current list. If all your zones are within two or three hours of each other the spacing is tighter. Add a zone from the opposite side of the world to stretch the scale and make small differences easier to see.

Can I copy the results to share with a colleague?

Yes. The "Copy summary" button writes a plain-text block to your clipboard with each zone, date, time and UTC offset on its own line. Paste it directly into a chat message, calendar invite or email.

Time Zone Difference Calculator

Coordinating across cities has always meant mental arithmetic — “they are UTC-5, it is 3 pm here so that is… 8 am there?” — and that arithmetic goes wrong as soon as daylight saving shifts one country but not another. This calculator removes the guesswork entirely.

Add any combination of cities and regions from a list of more than 70 IANA time zones, and the tool immediately shows the live local time in each one, the UTC offset at this exact instant (DST-aware), and the signed hour-and-minute gap relative to the first zone in your list. Switch to “custom date/time” mode and you can instead anchor the display to any moment you choose, interpreted in whichever zone that time belongs to — perfect for planning a meeting next Tuesday or checking what a deadline means across time zones.

How it works

The tool uses the browser’s built-in Intl.DateTimeFormat API, which ships with a complete IANA time-zone database. No external API calls are made. Two calls power most of the display:

Wall clock — Intl.DateTimeFormat with hour, minute, second and the target timeZone option returns the local time in that zone for any given Unix timestamp.
UTC offset — the same API called with timeZoneName: "shortOffset" returns a string like GMT+5:30 or GMT-8. The calculator parses the sign, hours and minutes from that string and formats it as UTC+05:30 or UTC-08:00.

The “diff from first” column is simple subtraction: offset(zone N) - offset(zone 1). Because both offsets are computed from the same UTC epoch, the difference is valid even when the two zones are on opposite sides of a DST boundary.

The SVG offset diagram maps each zone’s UTC offset to a horizontal position on a shared track. The leftmost point is the most-negative offset (furthest west) and the rightmost is the most-positive (furthest east). Each zone’s current wall-clock time is labelled beneath its tick mark, so you can glance at the diagram and immediately see the spread.

Example: scheduling a call between London, New York and Tokyo

Suppose it is 10:00 on a Wednesday in London (BST, UTC+1) and you want to find a slot that works for all three cities.

City	UTC offset (summer)	10:00 London =
London	UTC+1	10:00 Wed
New York	UTC-4	05:00 Wed
Tokyo	UTC+9	18:00 Wed

A 10 am London call is 5 am in New York — too early. Push it to 14:00 London and New York sees 09:00, Tokyo sees 22:00. A 22:00 end-of-day call works for Tokyo if the meeting is short. The tool shows all three columns simultaneously so you can slide the custom time until every city lands in a reasonable window.

Formula note

The offset in minutes for any IANA zone Z at Unix epoch E is extracted from Intl.DateTimeFormat("en-US", { timeZone: Z, timeZoneName: "shortOffset" }). The returned GMT string is parsed as:

offset_minutes = sign * (hours * 60 + minutes)

where sign is +1 for zones east of UTC and -1 for zones west. The hour difference between two zones A and B is then:

diff_hours = (offset_A - offset_B) / 60

All arithmetic is performed in integer minutes to avoid floating-point rounding errors when zones like India (UTC+5:30) or Newfoundland (UTC-3:30) are involved.