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 places, this offset changes twice a year because of Daylight Saving Time (DST). For example, Adelaide is UTC+9:30 in winter and UTC+10:30 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 not, because 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 Time, China Standard Time, Cuba Standard Time, and more.

A UTC offset always indicates a clear, fixed relationship to UTC.

What is the difference between GMT and UTC?

For most everyday purposes, GMT and UTC are the same, since both represent UTC+0. The difference is technical:

• GMT is the older term, based on the average solar time at the Royal Observatory in Greenwich, London.
• UTC is the current global standard, introduced 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 DST, a location may use a different time zone name and a different UTC offset. For example, Adelaide switches from ACST (UTC+9:30) in winter to ACDT (UTC+10:30) in summer. Not all places observe DST. Some regions, such as most of Arizona and Hawaii in the United States, stay on the same time all year.

Does Adelaide observe Daylight Saving Time?

Yes. Clocks move forward on the first Sunday in October at 3:00 AM, and move back on the first Sunday in April at 2:00 AM.

What time zone is Adelaide in?

Adelaide is in the Australian Central Standard Time (ACT).

What is the difference between ACST and ACDT?

These are the two seasonal versions of Australian Central Standard Time:

• ACST (UTC+9:30), which stands for " Australian Central Standard Time," is used in winter.
• ACDT (UTC+10:30), which stands for "Australian Central Daylight Time," is used in summer.

What is an IANA time zone identifier?

IANA (Internet Assigned Numbers Authority) time zone identifiers are the official, permanent names used by computers to handle time correctly, including DST rules. They use the format Continent/City, such 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 Adelaide is Australia/Adelaide .