How do time zones work?

The world is divided into 24 primary time zones, each roughly 15 degrees of longitude wide. UTC (Coordinated Universal Time) is the reference point. Zones ahead of UTC add hours (e.g., UTC+9 for Tokyo); zones behind subtract hours (e.g., UTC-5 for New York EST).

UTC (Coordinated Universal Time) is the global time standard. It replaced GMT as the reference and does not observe daylight saving time. All other time zones are defined as offsets from UTC (e.g., EST is UTC-5, CET is UTC+1).

What is the difference between EST and EDT?

EST (Eastern Standard Time) is UTC-5 and used during winter. EDT (Eastern Daylight Time) is UTC-4 and used during summer daylight saving time. This converter automatically accounts for DST based on the date you select.

How do I find a meeting time that works across time zones?

Use the converter to check what a proposed time looks like in each participant's time zone. Look for overlap in normal business hours (9 AM – 6 PM). The comparison view shows all selected cities side by side to find the best window.

Does this converter handle daylight saving time?

Yes. It uses the Intl.DateTimeFormat API built into your browser, which automatically applies the correct DST rules for each time zone based on the date. Results are always accurate for the specific date and time you enter.

Time Zone Converter – Free Online World Clock & Time Converter

Understanding Time Zones

Time zones were established in the late 19th century to standardize time across regions. Before that, every city set its own local time based on the sun's position. The introduction of railroads and telegraphs made synchronized timekeeping essential. Today, the world uses UTC (Coordinated Universal Time) as the global reference point.

How Time Zones Work

The Earth is divided into 24 primary time zones, each approximately 15 degrees of longitude wide (360° ÷ 24 = 15°). UTC sits at the Prime Meridian (0° longitude, through Greenwich, London). Zones to the east add hours; zones to the west subtract them. Some regions use half-hour or 45-minute offsets (India is UTC+5:30, Nepal is UTC+5:45).

Major Time Zones

Abbreviation	Name	UTC Offset
PST / PDT	Pacific (US)	UTC−8 / UTC−7
MST / MDT	Mountain (US)	UTC−7 / UTC−6
CST / CDT	Central (US)	UTC−6 / UTC−5
EST / EDT	Eastern (US)	UTC−5 / UTC−4
GMT / BST	UK	UTC±0 / UTC+1
CET / CEST	Central Europe	UTC+1 / UTC+2
IST	India	UTC+5:30
CST	China	UTC+8
JST	Japan	UTC+9
AEST / AEDT	Australia Eastern	UTC+10 / UTC+11

Daylight Saving Time (DST)

Many countries shift their clocks forward by one hour in spring and back in fall to extend evening daylight. This means time zone offsets change twice a year for those regions. Not all countries observe DST — Japan, China, India, and most of Africa and Southeast Asia do not. This converter automatically handles DST based on the date you select.

Example: New York is EST (UTC−5) in winter and EDT (UTC−4) in summer. London is GMT (UTC±0) in winter and BST (UTC+1) in summer. The difference between them is usually 5 hours, but during the weeks when one has changed clocks and the other hasn't, it can be 4 or 6 hours.

Tips for Working Across Time Zones

When scheduling meetings with international teams, aim for the overlap in business hours — typically 9 AM to 6 PM local time. For teams spanning the US and Europe, late morning US / afternoon Europe usually works best. For US and Asia-Pacific, early morning US / evening Asia is often the only viable window. Always specify the time zone explicitly when sending meeting invitations.

Frequently Asked Questions

The world has 24 primary zones, each ~15° of longitude. UTC is the reference. Zones east of UTC add hours (e.g., Tokyo UTC+9), zones west subtract (e.g., New York UTC−5).
UTC (Coordinated Universal Time) is the global time standard. It doesn't observe DST. All other zones are defined as offsets from UTC. It effectively replaced GMT as the reference.
EST (Eastern Standard Time) is UTC−5 used in winter. EDT (Eastern Daylight Time) is UTC−4 used in summer. This converter handles the switch automatically based on your selected date.
Use the converter to check what a proposed time looks like in each person's zone. Look for overlap in 9 AM–6 PM business hours. The world clock grid shows all major cities at once.
Yes. It uses the browser's Intl.DateTimeFormat API, which applies the correct DST rules for each zone based on the specific date you enter. Results are always date-accurate.
Some countries chose offsets that better match their geographic position or for political reasons. India uses UTC+5:30, Nepal uses UTC+5:45, and parts of Australia use UTC+9:30. These are fully supported by this converter.