Time Zone Basics: UTC, IANA Names & How Offsets Work
A clear guide to UTC offsets, IANA time zone identifiers, and why abbreviations like EST can be ambiguous.
What Is UTC?
Coordinated Universal Time (UTC) is the primary time standard by which the world regulates clocks and time. It is the successor to Greenwich Mean Time (GMT) and was adopted in 1960 by the International Telecommunication Union. While GMT is defined by the position of the sun relative to the Prime Meridian in Greenwich, England, UTC is based on International Atomic Time with leap seconds added to keep it within 0.9 seconds of solar time.
Every time zone in the world is defined as a positive or negative offset from UTC. For example, UTC+5:30 means 5 hours and 30 minutes ahead of UTC -- when it is 12:00 PM UTC, it is 5:30 PM in that zone. UTC-5 means 5 hours behind UTC -- when it is 12:00 PM UTC, it is 7:00 AM in that zone.
UTC itself does not observe Daylight Saving Time. It is always the same, which makes it the ideal reference point for scheduling across time zones. When in doubt, express times in UTC and let each participant convert to their local time.
What Are IANA Time Zone Names?
The IANA (Internet Assigned Numbers Authority) Time Zone Database -- also known as the "tz database" or "Olson database" -- assigns a unique identifier to every time zone in the world using a Continent/City format. For example:
America/New_York-- Eastern Time (US)Asia/Kolkata-- India Standard TimeEurope/Berlin-- Central European TimeAustralia/Sydney-- Australian Eastern Time
IANA names exist because abbreviations are ambiguous. "CST" could mean Central Standard Time (US, UTC-6), China Standard Time (UTC+8), or Cuba Standard Time (UTC-5). "IST" could mean India Standard Time (UTC+5:30), Irish Standard Time (UTC+1), or Israel Standard Time (UTC+2). Using the full IANA identifier eliminates any confusion.
IANA names are also essential for programming. Every major programming language and operating system uses the tz database to handle time zone conversions, DST transitions, and historical offset changes. When you set a timezone in JavaScript, Python, or your calendar app, you are using IANA names under the hood.
How UTC Offsets Work
A UTC offset tells you how many hours and minutes a time zone is ahead of or behind Coordinated Universal Time. Positive offsets (UTC+) are east of the Prime Meridian, and negative offsets (UTC-) are west.
Most offsets are whole hours, but several are not. India is UTC+5:30, Nepal is UTC+5:45, and the Chatham Islands are UTC+12:45. These half-hour and 45-minute offsets exist for historical and geographical reasons.
| Abbreviation | Time Zone | Current Offset |
|---|---|---|
| ET | Eastern Time (ET) | UTC-5 |
| CT | Central Time (CT) | UTC-6 |
| PT | Pacific Time (PT) | UTC-8 |
| GMT | Greenwich Mean Time (GMT) | UTC±0 |
| CET | Central European Time (CET) | UTC+1 |
| IST | India Standard Time (IST) | UTC+5:30 |
| JST | Japan Standard Time (JST) | UTC+9 |
| AEST | Australian Eastern Time (AEST) | UTC+11 |
| UTC | Coordinated Universal Time (UTC) | UTC±0 |
Offsets reflect the current DST state at build time and may differ during DST transitions.
What Is Daylight Saving Time?
Daylight Saving Time (DST) is the practice of advancing clocks by one hour during warmer months to extend evening daylight. In spring, clocks "spring forward" (e.g., 2:00 AM becomes 3:00 AM), and in autumn, they "fall back" (e.g., 2:00 AM becomes 1:00 AM). This means a time zone's UTC offset changes twice a year -- for example, US Eastern Time shifts between UTC-5 (EST) in winter and UTC-4 (EDT) in summer.
Not all time zones observe DST. Many countries near the equator see little variation in day length and have no need for clock changes. Even among countries that do observe DST, the start and end dates vary -- the US, EU, and Australia all switch on different weekends.
Current DST Status
Status computed at build time. For a deep dive into DST rules, transition dates, and team impact, see our complete DST guide.
Common Time Zone Abbreviations
Time zone abbreviations are shorthand labels used in everyday conversation and email signatures. Here are the ones you will encounter most often when coordinating across regions.
| Abbreviation | Full Name | Region |
|---|---|---|
| ET | Eastern Time | US East Coast |
| CT | Central Time | US Midwest |
| PT | Pacific Time | US West Coast |
| GMT | Greenwich Mean Time | UK (winter) |
| CET | Central European Time | Most of mainland Europe |
| IST | India Standard Time | India (no DST) |
| JST | Japan Standard Time | Japan (no DST) |
This is just the most common subset. For a complete reference table with all 28 tracked abbreviations, IANA names, offsets, and DST status, see the full Time Zone Abbreviations reference.
Why Time Zones Matter for Teams
If you are on a distributed team, time zones are not just trivia -- they directly affect your ability to collaborate. A poorly scheduled meeting can mean someone joins at 11 PM their time, or that half the team misses a standup because DST shifted the meeting by an hour without anyone noticing.
Understanding the basics covered on this page -- UTC as the universal reference, IANA names for precision, and DST as a source of offset changes -- gives you the foundation to schedule meetings that respect everyone's working hours.
Next Steps
- Scheduling Across Time Zones -- Proven strategies for finding overlap, rotating meeting times fairly, and adopting async-first practices.
- Meeting Time Finder -- Input your team's time zones and instantly see overlapping work hours.
- Quick Time Zone Converter -- Convert any time between two zones with live DST handling.
- Best Meeting Time Calculator -- Get ranked meeting slots scored for fairness, mid-day preference, and DST safety.