Basics · Reference

Time zone glossary

Short, exact definitions of the terms used across this site, each linked to the page that explains it in depth.

Terms defined
33
Eastern Time offset now
UTC−5 / UTC−4
Reference zone
America/New_York
Next clock change
Second Sunday in March

Why does precise time zone terminology matter?

Many scheduling mistakes are vocabulary mistakes. "3 PM EST" in July names an offset, UTC−5, that New York is not using, so a literal reader arrives at 4 PM EDT, an hour late. ET is the zone, EST and EDT its two offsets, and only an identifier such as America/New_York tells software which rule applies on a given date. Each term can be linked directly, for example /glossary/#utc; the live clock is on the Eastern Time homepage.

What do the time zone terms A–L mean?

Ambiguous (repeated) hour
The hour that occurs twice when clocks fall back. On November 7, 2027, 1:00–1:59 AM happens first in EDT, then again in EST, so "1:30 AM" that night needs an offset.
America/New_York
The IANA identifier for Eastern Time in the United States, recording every change since 1883. Ontario and Quebec use America/Toronto, with the same rules today.
Apparent solar time
Time read from the real Sun, as on a sundial; it drifts against the clock during the year (see equation of time).
Civil time
The legal time a jurisdiction uses for daily life: standard time or daylight time as set by law.
Daylight saving time (DST)
Clocks set one hour ahead in summer. Eastern Time observes it from the second Sunday in March to the first Sunday in November; dates are in our daylight saving time guide.
Energy Policy Act of 2005
The law whose Section 110 created today's March–November DST schedule, starting in 2007.
Equation of time
The gap between apparent and mean solar time. A sundial runs up to about 16 minutes fast in early November and about 14 minutes slow in mid-February.
EST5EDT
A legacy POSIX-style zone name: "EST, five hours behind UTC, with EDT". The rule string EST5EDT,M3.2.0,M11.1.0 encodes the current US dates; new code should use America/New_York.
ET, EST and EDT
ET is the zone in any season; EST (UTC−5) is its winter offset and EDT (UTC−4) its summer one. Full comparison: EST vs EDT.
Etc/GMT+5
A fixed UTC−5 zone with no DST. Its sign is inverted by POSIX convention, so +5 means five hours behind UTC. Places on that offset all year are listed under UTC−5.
GMT (Greenwich Mean Time)
Mean solar time at the Royal Observatory, Greenwich, the world reference chosen in 1884. Today it is the UK's winter civil time and, informally, another name for UTC+0. See EST to GMT.
IANA time zone database (tz)
The public-domain record of every zone's offsets and DST rules, reliable from 1970 and often much earlier, maintained by volunteers under IANA. Operating systems, browsers and this site's clocks use it.
ISO 8601
The international date-time format. 2026-11-07T14:00:00-05:00 is 2:00 PM EST on November 7, 2026; a trailing Z means UTC.
Leap second
An extra second added to UTC to keep it within 0.9 s of Earth's rotation; 27 were inserted from 1972 to 2016. Under a 2026 international decision, UTC runs without them from May 20, 2027.

What do the time zone terms M–Z mean?

Mean solar time (local mean time)
Time from an imaginary Sun moving at constant speed, so every day lasts 24 hours. Each longitude has its own; New York's was UTC−4:56:02 until 1883 (see the history of Eastern Time).
Meridian
A line of constant longitude from pole to pole. Every 15° equals one hour; Eastern Time is based on the 75th meridian west.
Military time zone letters
One letter per whole-hour offset: Z (Zulu) is UTC, R (Romeo) UTC−5 for EST, Q (Quebec) UTC−4 for EDT and J (Juliett) local time. See military time.
NIST and time.gov
The National Institute of Standards and Technology keeps the US time scale UTC(NIST) and broadcasts it by radio (WWV, WWVB) and online at time.gov.
Prime meridian
Longitude 0°, through Greenwich, adopted at the 1884 International Meridian Conference. The modern GPS reference meridian lies about 100 metres east.
Skipped hour
The hour that does not exist when clocks spring forward. On March 14, 2027, Eastern clocks jump from 1:59:59 AM EST to 3:00:00 AM EDT, so 2:30 AM never happens.
Solar noon
The moment the Sun crosses the local meridian and stands highest. Inside Eastern Time it ranges from before 11:20 AM EST in Gaspé, Quebec, to about 2:00 PM EDT in western Michigan; see time zone boundaries.
Spring forward, fall back
The DST reminder: clocks go ahead one hour at 2:00 AM on the second Sunday in March and back at 2:00 AM on the first Sunday in November.
Standard time
A zone's base offset without DST. Federal law (15 U.S.C. §261) defines Eastern standard time as UTC "retarded by 5 hours", a clock railroads first kept in 1883.
Sunshine Protection Act
A federal bill to make DST permanent nationwide, with a state opt-out. The House passed it (H.R. 139) 308–117 on July 14, 2026; as of November 2026 the Senate had not voted and it sits with the Senate Commerce Committee, so clock changes continue.
TAI (International Atomic Time)
A continuous scale computed by the International Bureau of Weights and Measures from hundreds of atomic clocks; without leap seconds it has run 37 s ahead of UTC since January 1, 2017.
Time zone abbreviation
A short label such as EST, EDT or ET. Abbreviations are not unique (CST can mean US Central, China or Cuba), so software relies on zone identifiers. Decoded on our abbreviations page.
Time zone boundary
The legal line between zones. In the US the Department of Transportation sets it in 49 CFR Part 71, along county lines and rivers rather than meridians.
Uniform Time Act
The 1966 law that made DST dates the same nationwide and let a state opt out only as a whole; the Department of Transportation administers its zones.
Unix time (epoch)
Seconds counted from 00:00:00 UTC on January 1, 1970, ignoring leap seconds. In New York that moment was 7:00 PM EST on December 31, 1969.
UTC (Coordinated Universal Time)
The world's reference time scale, kept by atomic clocks and steered toward Earth's rotation. Every offset is measured from it; conversions are on EST to UTC.
UTC offset
The difference between local time and UTC, written UTC−5 or −05:00. Eastern Time alternates between −05:00 and −04:00; see the UTC offset guide.
Zone identifier
An IANA name in Area/Location form, such as America/Indiana/Indianapolis. Unlike an abbreviation it is unique and carries the place's full history of rule changes.
Zulu time
The military and aviation name for UTC, from the letter Z. 1900Z is 2:00 PM EST or 3:00 PM EDT.

Frequently asked questions

Which time label should I put in a meeting invite?

Write the local time with ET or the city, let your calendar attach the zone America/New_York, and add UTC for international readers, for example 2:00 PM ET (19:00 UTC) on November 7, 2026.

How often does the time zone database change?

Several times a year: 2026 brought six releases by late October (2026a–2026f), mostly for Canadian provinces that stopped changing clocks. Outdated devices can show the wrong local time.

What is the difference between a time zone and a UTC offset?

An offset is one number at one instant, such as −05:00. A time zone is a place's rules over time: America/New_York uses two offsets each year, so storing only −05:00 loses information as soon as DST begins.

Do leap seconds affect Eastern Time?

Yes, because Eastern Time is defined as an offset from UTC. A leap second added at 23:59:60 UTC fell at 6:59:60 PM EST on December 31 or 7:59:60 PM EDT on June 30. The last one came on December 31, 2016.

Explore

EST vs EDT

The most misused pair of terms, explained with dates.

UTC offset

How −05:00 and −04:00 are written and used.