Last updated: April 2026 · Reviewed by Calculatormatics Editorial Team

Time Zone Converter — Compare Up to 4 Zones Side by Side

Convert any date and time between 30+ world time zones instantly. The converter uses your browser's built-in Intl.DateTimeFormat API, which automatically handles Daylight Saving Time (DST) for every zone — no manual offset lookup required. Enable the Meeting Planner to see a 24-hour business-hours strip for up to four zones side by side: green cells mark 9 am–6 pm local, red marks night hours. Half-hour offsets like India's UTC+5:30 and quarter-hour offsets like Nepal's UTC+5:45 are handled correctly. Choose a source zone, pick a date and time (or use current time), select your target zone, and the converter shows the converted time, UTC offsets, the time difference between zones, and the day of week at the destination.

Time difference (target vs source): +5h
Source offset: UTC-04:00
Target offset: UTC+01:00

How Time Zones Work

Every location on Earth is assigned a UTC offset — the number of hours and minutes ahead of or behind Coordinated Universal Time (UTC). UTC is the root reference: it never changes, never observes daylight saving, and is maintained by atomic clocks operated by international standards bodies.

Most time zones are whole-hour offsets: UTC-8, UTC+1, UTC+9. But some are not. India is UTC+5:30, Iran is UTC+3:30, Nepal is UTC+5:45, and the Chatham Islands (New Zealand) are UTC+12:45. These non-standard offsets arose from political and practical decisions made when time standardization was first adopted.

Time zones are identified in software using IANA identifiers in the format Continent/City — for example, America/New_York, Europe/London, Asia/Kolkata. The city is the representative city for that rule set, not a geographic boundary. These identifiers avoid ambiguity: "EST" could mean Eastern Standard Time (US, UTC-5), Eastern Standard Time (Australia, UTC+10), or Eastern Standard Time (India, historically). IANA identifiers are unambiguous.

Common abbreviations and their IANA equivalents:

Daylight Saving Time

Daylight Saving Time (DST) shifts the local clock forward by one hour in spring and back in autumn, effectively moving an hour of daylight from the morning to the evening. This means the UTC offset for a DST-observing zone is not constant throughout the year.

Key facts about DST:

• US and Canada: DST starts the second Sunday of March and ends the first Sunday of November.
• European Union: DST starts the last Sunday of March and ends the last Sunday of October.
• Australia and New Zealand: DST runs roughly October to April (Southern Hemisphere spring/summer).
• No DST: Most of Asia (Japan, China, India, Singapore), most of Africa, most of the Middle East, and US state Arizona do not observe DST.

Because the US and EU switch on different dates, there are roughly 2-week windows twice a year when the US-to-Europe offset is one hour different from normal. For example, New York is normally 5 hours behind London (UTC-4 vs UTC+1 in summer), but in mid-March — after the US has switched but before the EU does — New York is only 4 hours behind London.

This converter handles DST correctly because it uses the browser's IANA database rather than a static offset. When you pick a specific date, the converter looks up the correct offset for that date in each zone.

Half-Hour and Quarter-Hour Offsets

Not every time zone sits on a clean hourly boundary. Several large countries deliberately chose offsets that place them between standard hour marks:

These offsets are historical rather than irrational — they reflect where those countries fell geographically relative to the sun's position at noon, or were set to distinguish a nation from its neighbours. Nepal's UTC+5:45 is the only quarter-hour offset currently in regular use.

Worked Example: Scheduling a London–Tokyo–San Francisco Meeting

Suppose you want to schedule a 1-hour call with colleagues in San Francisco, London, and Tokyo. Here is what 10:00 am PST (America/Los_Angeles, UTC-8) looks like in all three cities on a typical winter weekday:

Time reference: 10:00 am PST (America/Los_Angeles, UTC-8)

San Francisco   10:00  Monday  PST (UTC-8)  ← source
London          18:00  Monday  GMT (UTC+0)  ← 8 hours ahead
Tokyo           03:00  Tuesday JST (UTC+9)  ← 17 hours ahead

Tokyo at 3:00 am is not workable. To find a better overlap:

Tokyo 09:00 = London 01:00 = San Francisco 17:00  (previous day issue)
Tokyo 17:00 = London 09:00 = San Francisco 01:00  (SF asleep)

Best compromise: London 08:00 = San Francisco 00:00 = Tokyo 17:00
  → SF must work at midnight; this is the fundamental problem
    of scheduling across a 17-hour gap.

Realistic solution: record async video or rotate meeting times
between weeks so no single team always takes the overnight slot.

The meeting planner strip in this converter shades cells green (9 am–6 pm local), yellow (7–9 am / 6–9 pm), and red (night) so you can visually spot the overlap — or the absence of one.

The IANA (tz) Database

The IANA Time Zone Database (also called zoneinfo or the Olson database, after its original maintainer Arthur David Olson) is the canonical reference for time zone rules worldwide. It records every time zone rule change since 1970, plus older historical data for many zones. When a country changes its DST rules or shifts its UTC offset — as Samoa did in 2011 when it skipped Friday 30 December entirely to move from UTC-11 to UTC+13 — the IANA database is updated, and operating systems and browsers pick up the change in their next update.

The database is updated roughly 5–10 times per year as new decisions are announced. Major consumers include Linux, macOS, Windows (via their own conversion), Java, Python, JavaScript (via Intl.DateTimeFormat), PostgreSQL, MySQL, and virtually every web browser. When you ask your browser to convert a time in America/New_York, it uses this database.

Common Scheduling Pitfalls

Even experienced engineers make these mistakes when working across time zones:

• DST transition days have 23 or 25 hours. When clocks spring forward, one hour is skipped: 01:59:59 is followed by 03:00:00. Any meeting or cron job scheduled at 02:30 that day will either be missed or fire at the wrong time, depending on the software.
• Recurring events in local time vs UTC. A recurring meeting at "every Monday 09:00 America/New_York" shifts by one hour in UTC twice a year. If stored as "09:00 UTC-5" instead of using the IANA identifier, it will be wrong half the year.
• Ambiguous "fall back" hour. When clocks fall back (e.g., 02:00 becomes 01:00), 01:30 occurs twice. Without a UTC timestamp, it is impossible to know which occurrence is meant.
• Confused AM/PM on 12-hour systems. Midnight is 12:00 AM and noon is 12:00 PM, which most people find counterintuitive. Using 24-hour (military) time eliminates this class of error.
• Assuming all zones differ by whole hours. If you manually add/subtract hours to convert a time, you will get the wrong answer for India (+5:30), Iran (+3:30), or Nepal (+5:45). Always use a library or API that consults the IANA database.
• Server and database time zones. Databases store timestamps as UTC internally but can display them in a local time zone. If the server's system time zone is set incorrectly, timestamps will appear wrong. Always configure server infrastructure to UTC.

Business Hours Visualizer

The Meeting Planner in this converter adds a 24-column strip for each selected zone. Each column represents one UTC hour (00–23), and the cell shows the corresponding local hour in that zone. The color coding is:

• Green: 09:00–17:59 local — standard business hours
• Yellow: 07:00–08:59 or 18:00–20:59 local — early/late but workable
• Red: 21:00–06:59 local — nighttime, unsuitable for meetings

To find a good meeting slot, look for a UTC column where all participating zones show green or yellow. The strip is based on the calendar date of your selected reference time (or the current date if "use current time" is checked), so DST is automatically factored in for that day.

As a practical example, the London–Singapore overlap is relatively generous: Singapore business hours (09:00–18:00 SGT = 01:00–10:00 UTC) overlap with London's 09:00–18:00 BST (08:00–17:00 UTC) for about two hours (09:00–10:00 UTC = 17:00 London / 17:00 Singapore). That narrow window is early evening for London and end-of-day in Singapore — manageable for weekly syncs.

Frequently Asked Questions