Time Zones: How Longitude Becomes Clock Time, and Why Eastern Standard Time Is UTC Minus 5
Earth turns 15 degrees of longitude every hour, and time zones were built on that. U.S. Eastern Standard Time is five hours behind UTC, written UTC minus 5.
Try it in the instrument
Set the sky to U.S. Eastern Standard Time, as an offset from UTC.
Also called
Zone time (U.S. Naval Observatory), Standard time zones
A simplified model
The drawings are computed from the numbers in the sources. They simplify the real sky. For exact times and dates, use official tables.
At the same moment, a clock in the Eastern time zone and a clock in the Pacific time zone show different hours. That is not a mistake. It is the whole idea of time zones: clocks in one band of the world share an hour, and the next band over is an hour away.
This guide explains how longitude turns into clock time, how the U.S. zones are measured from UTC, and why zone borders do not run in straight lines. It ends with a short task in the Clock Dial.
Why the world has time zones
Before standard time, time of day was a local matter. The U.S. Naval Observatory says most cities and towns used some form of local solar time, kept by a well-known clock such as one on a church steeple or in a jeweler's window. Standard time in time zones was instituted in the U.S. and Canada by the railroads on November 18, 1883. NIST dates the railroads' first zones to the 1880s and says that within several years time zones had spread internationally.
The law came later. The Observatory notes that standard time was not established in U.S. law until the Act of March 19, 1918, sometimes called the Standard Time Act.
Fifteen degrees per hour
Earth rotates through 360 degrees in 24 hours, so one hour of time difference equals 15 degrees of longitude. Royal Museums Greenwich and NOAA both state it that way. Put another way, the Earth turns through one degree of longitude every four minutes.
NIST says this was the basis of time zones: divide the Earth into sections of longitude 15 degrees each, which allows 24 zones to stand for the 24 hours of a day. In the ideal layout, described in NIST's glossary, each zone is 15 degrees wide and centered on a central meridian, counted from the prime meridian at 0 degrees longitude. The central meridian of each zone has a longitude divisible by 15. When the Sun is directly above it, local time across the whole zone would be noon. The guide to solar noon follows that idea further.
The reference clock is Coordinated Universal Time, or UTC. NASA's Marshall Space Flight Center calls UTC the international time standard and says zero hours UTC is midnight in Greenwich, England, on the zero meridian. Its worked example is Huntsville, Alabama, at about 90 degrees west. Local time there lags UTC by 6 hours, because 90 ÷ 15 = 6, assuming Central Standard Time. In Central Daylight Time the lag is 5 hours. NASA's rule for the sign is to subtract time in west longitudes and add time in east longitudes.
| U.S. zone | Standard time from UTC | Hours × 15 degrees |
|---|---|---|
| Eastern | −5 hours | 75 degrees west |
| Central | −6 hours | 90 degrees west |
| Mountain | −7 hours | 105 degrees west |
| Pacific | −8 hours | 120 degrees west |
The offsets in the middle column come from NIST. The last column is our own multiplication, the ideal central meridian of each zone.
The U.S. zones and their offsets
The Naval Observatory says standard time in the U.S. and its territories is observed within nine time zones. Standard time in each zone is a whole number of hours offset from UTC. To get U.S. civil time from UTC, you use the Observatory's conversion table, which says how many hours to subtract, or add in one case.
| Zone | Standard time | Daylight time |
|---|---|---|
| Atlantic | AST, UTC−4 | None listed |
| Eastern | EST, UTC−5 | EDT, UTC−4 |
| Central | CST, UTC−6 | CDT, UTC−5 |
| Mountain | MST, UTC−7 | MDT, UTC−6 |
| Pacific | PST, UTC−8 | PDT, UTC−7 |
| Alaska | AKST, UTC−9 | AKDT, UTC−8 |
| Hawaii-Aleutian | HST, UTC−10 | HDT, UTC−9 |
| Samoa | SST, UTC−11 | None listed |
| Chamorro | ChST, UTC+10 | None listed |
Chamorro Standard Time is the odd one out. The Observatory says to add 10 hours to UTC for it, while the other eight zones subtract.
Eastern Standard Time in detail
The Observatory's table says to subtract 5 hours from UTC to get Eastern standard time, or EST. Eastern daylight time, EDT, subtracts 4. NIST's table agrees: EST −5 and EDT −4.
The Observatory gives two conversions as examples. 11:00 UTC is 5:00 CST, which is Central standard time, 6 hours behind. And 10 March at 02:00 UTC is 9 March at 21:00 EST. The second one shows why the Observatory reminds readers to take dates into account: subtracting 5 hours from 02:00 crosses midnight and lands on the day before.
Daylight saving time changes the local offset but not UTC. NIST says daylight saving time begins each year on the second Sunday in March at 2 a.m. local time, and standard time returns on the first Sunday in November at 2 a.m. UTC itself does not observe daylight saving time, NIST adds, so converting to or from local time means knowing whether daylight time is in effect. The Department of Transportation, not NIST, oversees and regulates daylight saving time.
Why zone borders bend
NIST says that in practice the boundaries between time zones are often modified to fit political boundaries, and a few countries use a local time that differs by half an hour from that of the central meridian. In the United States, borders were usually drawn along state lines, natural landmarks and county lines, and a few counties are split between two time zones.
The Department of Transportation is responsible for the U.S. zone boundaries, the Naval Observatory says, and the principal standard for a change is the "convenience of commerce." So the 15-degree strips are a starting plan, not a map of today's borders.
Common mistakes
- Saying Eastern Standard Time is UTC plus 5. Clocks in the Eastern zone run behind UTC. The Observatory's rule is to subtract 5 hours from UTC, so the offset is minus 5.
- Mixing up EST and EDT. Standard time is UTC minus 5 and daylight time is UTC minus 4. UTC itself never changes.
- Forgetting the date. At 02:00 UTC on 10 March, it is still 9 March in the Eastern zone, at 21:00 EST.
- Expecting straight 15-degree strips on a map. That is the ideal layout. Real borders follow state lines, landmarks and county lines.
- Assuming every zone is a whole hour. NIST notes that a few countries use a local time half an hour away from the central meridian.
Try it in the instrument
Open the Clock Dial and set the sky to U.S. Eastern Standard Time, as an offset from UTC. The target is minus 5 hours, which means the clocks run five hours behind UTC.
The relation behind it is the 15-degree rule. Five hours of Earth's turning is 5 × 15 = 75 degrees of longitude, and the Eastern zone sits west of Greenwich, so by NASA's sign rule you subtract. That gives UTC minus 5. Two wrong answers are tempting: minus 4 is Eastern daylight time, and plus 5 would put the clocks east of Greenwich.
Questions people ask
What is U.S. Eastern Standard Time in UTC?
UTC minus 5 hours. The Naval Observatory's table says to subtract 5 hours from UTC to get Eastern standard time, and NIST lists it as EST −5.
How many degrees of longitude is one time zone?
In the ideal layout, 15 degrees, so that 24 zones cover the 360 degrees of the Earth. Real zones vary because borders follow political lines, according to NIST.
Does UTC change with daylight saving time?
No. NIST says UTC does not observe daylight saving time. Local clocks move, so the Eastern offset goes from minus 5 in standard time to minus 4 in daylight time.
How many time zones does the United States have?
The Naval Observatory says standard time in the U.S. and its territories is observed within nine time zones. NIST lists them as Atlantic, Eastern, Central, Mountain, Pacific, Alaska, Hawaii-Aleutian, Samoa and Chamorro.
Sources
- U.S. Time Zones, U.S. Naval Observatory, Astronomical Applications Department. https://aa.usno.navy.mil/faq/us_tzones. Opened November 10, 2026.
- Local Time FAQs, NIST Time and Frequency Division. https://www.nist.gov/pml/time-and-frequency-division/local-time-faqs. Opened November 10, 2026.
- Time and Frequency from A to Z, Ti, NIST Time and Frequency Division. https://www.nist.gov/pml/time-and-frequency-division/popular-links/time-frequency-z/time-and-frequency-z-ti. Opened November 10, 2026.
- What is longitude?, NOAA National Ocean Service. https://oceanservice.noaa.gov/facts/longitude.html. Opened November 10, 2026.
- What is longitude?, Royal Museums Greenwich (Royal Observatory Greenwich). https://www.rmg.co.uk/stories/time/what-longitude. Opened November 10, 2026.
- NASA/MSFC/ESSD - Coordinated Universal Time, NASA Marshall Space Flight Center, Earth Science and Systems Department. https://weather.ndc.nasa.gov/utc.html. Opened November 10, 2026.
We opened each source ourselves on the date shown. The drawings are our own and were checked against the numbers in these sources.
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