What is UTC?

UTC (Coordinated Universal Time) is the world’s primary time standard. All time zones are defined by how many hours they are ahead of (+) or behind (–) UTC. In many placesthis offset changes twice a year because of Daylight Saving Time (DST). For exampleChicago is UTC-6 in winter and UTC-5 in summer. UTC is computed by the Bureau International des Poids et Mesures (BIPM) using data from about 450 atomic clocks in 85 national laboratories worldwide.

Why use UTC offsets instead of just timezone names?

UTC offsets are unambiguous. Time zone abbreviations like “EST” or “CST” are notbecause they are used in multiple unrelated regions. For example:

• “Eastern Time” exists in both North America (UTC-5/UTC-4) and Australia (UTC+10/UTC+11).
• “CST” can mean U.S. Central Standard TimeChina Standard TimeCuba Standard Timeand more.

What is the difference between GMT and UTC?

For most everyday purposesGMT and UTC are the samesince both represent UTC+0. The difference is technical:

• GMT is the older termbased on the average solar time at the Royal Observatory in GreenwichLondon.
• UTC is the current global standardintroduced in 1972. It is based on atomic clocks, making it more precise.

What is Daylight Saving Time (DST)?

Daylight Saving Time (DST) is when clocks are moved forward by one hour in spring ("spring forward") and moved back by one hour in fall ("fall back"). The purpose is to extend evening daylight. During DSTa location may use a different time zone name and a different UTC offset. For exampleChicago switches from CST (UTC-6) in winter to CDT (UTC-5) in summer. Not all places observe DST. Some regionssuch as most of Arizona and Hawaii in the United States, stay on the same time all year.

Does Chicago observe Daylight Saving Time?

Yes. Clocks move forward on the second Sunday in March at 3:00 AMand move back on the first Sunday in November at 1:00 AM.

What time zone is Chicago in?

Chicago is in the Central Standard Time (CT).

What is the difference between CST and CDT?

These are the two seasonal versions of Central Standard Time:

• CST (UTC-6)which stands for " Central Standard Time," is used in winter.
• CDT (UTC-5)which stands for "Central Daylight Time," is used in summer.

What is an IANA time zone identifier?

IANA (Internet Assigned Numbers Authority) time zone identifiers are the officialpermanent names used by computers to handle time correctlyincluding DST rules. They use the format Continent/Citysuch as America/New_York or Europe/London . These identifiers ensure that devices automatically use the correct offset and DST transitions for each region.

The IANA time zone identifier for Chicago is America/Chicago .