Time zone calculator,
with DST awareness.

Field guide

Understanding time zones, UTC offsets, and daylight saving time.

A time zone is a region of the globe that observes a uniform standard time for legal, commercial, and social purposes. Time zones are defined relative to Coordinated Universal Time (UTC): the primary world time standard — by adding or subtracting a fixed offset expressed in hours and minutes.

UTC and the prime meridian

UTC is the successor to GMT (Greenwich Mean Time) and is the basis for all civil timekeeping worldwide. It runs at the same rate as atomic time and does not observe Daylight Saving Time. The offset from UTC tells you how far ahead or behind a location is:

• UTC+0: London (winter), Reykjavik, Casablanca.
• UTC+5:30: Mumbai and Delhi. India uniquely uses a 30-minute offset. It's neither UTC+5 nor UTC+6.
• UTC+5:45: Kathmandu, Nepal. One of only two countries in the world with a 45-minute offset.
• UTC+9: Tokyo, Seoul. Japan does not observe DST.
• UTC+12 and UTC−12: The International Date Line runs near the 180° meridian. Countries on either side can be on the same calendar day or 24 hours apart.

Daylight Saving Time (DST)

Many countries advance their clocks by one hour during the summer months to shift the extra daylight into the evening. When DST is in effect, time zones shift their UTC offset. The critical practical consequences:

• New York is UTC−5 in winter (EST) and UTC−4 in summer (EDT). When New York "springs forward," the gap between New York and UTC+0 (London) shrinks from 5 to 4 hours.
• London is UTC+0 in winter (GMT) and UTC+1 in summer (BST). When both the US and UK are on DST, New York and London are 5 hours apart. But during the few weeks when only one has switched, the gap is 4 or 6 hours.
• Not all countries observe DST. Japan, China, India, Iceland, most of Africa, and much of Asia do not observe DST — their UTC offsets are constant year-round.

This is why it matters to use an IANA-database converter (like this one) rather than manually adding a fixed offset: the offset changes twice a year in DST-observing regions, and the switchover dates differ between countries.

The IANA timezone database

All modern operating systems and browsers maintain a copy of the IANA Time Zone Database (sometimes called the "Olson database"), which contains the full historical and current DST rules for every named timezone in the world. When this calculator converts "9:00 AM New York, July 15" to Tokyo, it uses the browser's built-in Intl API backed by the IANA database — automatically applying the correct summer/winter offsets.

IANA timezone identifiers follow the format Region/City: America/New_York, Asia/Tokyo, Europe/London. These are the canonical names used by programming languages, databases, and operating systems worldwide.

Date crossovers and the International Date Line

When you convert a time across a large timezone span, the calendar date in the target timezone may differ from the source date. Two common scenarios:

• Evening in New York → next morning in Tokyo. If it's 11:00 PM Tuesday in New York (UTC−4), it's 12:00 PM Wednesday in Tokyo (UTC+9) — 13 hours ahead, so already the next day.
• Morning in Sydney → previous evening in London.If it's 9:00 AM Thursday in Sydney (UTC+11 in summer), it's 11:00 PM Wednesday in London (UTC+0) — the previous day.

This calculator flags these crossovers with a +1 day or −1 day badge so you never accidentally schedule a 9 AM meeting for the wrong calendar day.

Practical tips for international scheduling

• Agree on UTC. For international meetings, send invitations in UTC (e.g., "Tuesday, 14:00 UTC") to eliminate ambiguity about DST. Most calendar apps convert UTC to local time automatically.
• Watch DST switch weeks. In late October and early November, the EU and US switch out of DST at different times, creating a temporary 1-hour shift in the gap between them.
• Include the date for cross-date-line calls.A meeting at 6 AM Monday Tokyo time may be Sunday afternoon in New York — confirming the day prevents confusion.
• Beware half-hour and 45-minute zones. India (+5:30), Afghanistan (+4:30), Iran (+3:30 / +4:30 with DST), and Nepal (+5:45) use non-integer offsets. Always use a timezone converter rather than manually adding whole hours.
• China uses a single timezone. Despite spanning 5 natural time zones, the entirety of China uses UTC+8 (China Standard Time). Xinjiang in the far west follows an unofficial "Xinjiang Time" (UTC+6) informally.

Time zones in programming and databases

A common mistake in software development is storing times as "local time" without a timezone annotation. Best practice:

• Store all timestamps in UTC in databases and APIs. Convert to local time only for display.
• Use full IANA timezone identifiers (e.g., America/New_York) rather than abbreviations like "EST" — abbreviations are ambiguous (EST could refer to Eastern US, Eastern Australian, or Eastern Brazilian Standard Time).
• Represent times as ISO 8601 with a UTC offsetin API payloads: 2024-05-14T09:00:00-04:00.
• Test your application around DST transitions(typically 2:00 AM local time on the switchover day). This is when the most timezone-related bugs surface.