What is a time zone?
A time zone is a region of the Earth that uses the same clock time. Earth rotates 360 degrees in about 24 hours. This gives us a simple model of 24 one-hour time zones, with about 15 degrees of longitude per hour. In the real world, time-zone boundaries do not always follow these lines exactly.
- When it is 12:00 noon in New York, it is 9:00 a.m. in Los Angeles.
- When it is 8:00 a.m. in Chicago, it is 6:00 a.m. in Denver.
- When it is 3:00 p.m. in London, it is 10:00 a.m. in Miami.
Time zones help people in different places agree on what time it is. Without them, noon would be at different clock times in every town.
Why do we need time zones?
We need time zones because the Sun does not shine on all parts of Earth at the same time. When it is daytime on one side of the planet, it is nighttime on the other side. Time zones keep clocks roughly in line with the Sun’s position, so noon is close to when the Sun is highest in the sky.
- In New York, the Sun rises around 6:00 a.m. Eastern Time.
- In California, the Sun rises around 6:00 a.m. Pacific Time, which is three hours later than Eastern Time.
- If we used one global time, people in California might have sunrise at 3:00 a.m. on the clock.
Time zones also help with travel, business, and communication. They make it easier to plan events across the country.
How are time zones measured?
Time zones are measured from the Prime Meridian, which is 0 degrees longitude and passes through Greenwich, England. The Prime Meridian is at 0° longitude. UTC is the international reference time used to describe time offsets around the world. GMT is an older time standard associated with Greenwich, England. Moving east from the Prime Meridian, each time zone is one hour later. Moving west, each time zone is one hour earlier.
- UTC+0: London, England
- UTC−5: New York (Eastern Standard Time)
- UTC−8: Los Angeles (Pacific Standard Time)
- UTC+1: Paris, France
The “+” and “−” signs show how many hours a place is ahead of or behind UTC. For example, UTC−5 means the place is 5 hours behind UTC.
Time zones in the United States
The United States uses six main time zones: Eastern, Central, Mountain, Pacific, Alaska, and Hawaii-Aleutian. Each zone is one hour earlier than the zone to its east. For example, when it is 3:00 p.m. Eastern, it is 2:00 p.m. Central, 1:00 p.m. Mountain, and 12:00 noon Pacific.
- Eastern Time (ET): New York, Miami, Atlanta
- Central Time (CT): Chicago, Dallas, New Orleans
- Mountain Time (MT): Denver, Salt Lake City, Phoenix
- Pacific Time (PT): Los Angeles, San Francisco, Seattle
Some states, like Arizona, do not use Daylight Saving Time, so their time compared to other zones can change during the year.
Converting time between zones
To convert time from one zone to another, find the time difference between the zones. Then add or subtract that many hours. If you are moving from east to west, subtract hours. If you are moving from west to east, add hours.
- It is 4:00 p.m. in New York (ET). What time is it in Chicago (CT)?
ET is 1 hour ahead of CT. Subtract 1 hour → 3:00 p.m. - It is 10:00 a.m. in Denver (MT). What time is it in Los Angeles (PT)?
MT is 1 hour ahead of PT. Subtract 1 hour → 9:00 a.m. - It is 2:00 p.m. in Los Angeles (PT). What time is it in New York (ET)?
PT is 3 hours behind ET. Add 3 hours → 5:00 p.m.
Always check if the zones are using Daylight Saving Time. In the summer, most U.S. zones move one hour forward, but they still keep the same difference between each other.
Using a time zone map
A time zone map shows the borders of time zones around the world. It helps you see which cities share the same time and how many hours apart they are. The lines on the map are not always straight because they follow country and state borders.
- Find New York and London on a map. New York is 5 hours behind London.
- Find Los Angeles and Tokyo. Los Angeles is 17 hours behind Tokyo.
- Find Chicago and Denver. Chicago is 1 hour ahead of Denver.
When using a time zone map, remember that crossing a time zone line changes the time by one hour. The direction you travel (east or west) tells you whether to add or subtract.
Solving word problems with time zones
Word problems about time zones often ask you to find the time in one place when you know the time in another. Follow these steps: 1) Identify the starting time and zone. 2) Identify the target zone. 3) Find the time difference. 4) Add or subtract to get the answer.
- Maya lives in Dallas (CT). She calls her cousin in Seattle (PT) at 6:00 p.m. CT. What time is it in Seattle?
CT is 2 hours ahead of PT. Subtract 2 hours → 4:00 p.m. - Liam is in Boston (ET). His soccer game starts at 1:00 p.m. ET. His grandparents watch in Denver (MT). What time do they watch?
ET is 2 hours ahead of MT. Subtract 2 hours → 11:00 a.m. - A flight leaves Los Angeles (PT) at 9:00 a.m. and lands in New York (ET) 5 hours later. What time is it in New York when the plane lands?
9:00 a.m. PT + 5 hours = 2:00 p.m. PT. Add 3 hours for ET → 5:00 p.m. ET.
Always label your answer with the correct time zone. For example, write “5:00 p.m. ET” instead of just “5:00 p.m.”
Time zones and the 24-hour day
Earth rotates once every 24 hours, which is why we have 24 time zones. Each time zone represents one hour of the day. When you cross a time zone line going east, you add one hour. When you cross going west, you subtract one hour.
- If it is 11:00 p.m. in New York, it is 8:00 p.m. in Los Angeles.
- If it is 7:00 a.m. in Denver, it is 9:00 a.m. in New York.
- If it is 12:00 noon in Chicago, it is 10:00 a.m. in Los Angeles.
The 24-hour day also explains why some places are a day ahead. For example, at some times, Tokyo is already on the next calendar day when it is still the previous day in New York.
Daylight Saving Time
Daylight Saving Time (DST) is a practice where clocks are set forward one hour in the spring and back one hour in the fall. This gives more daylight in the evening during warmer months. Not all places use DST.
- In March, clocks “spring forward” from 2:00 a.m. to 3:00 a.m.
- In November, clocks “fall back” from 2:00 a.m. to 1:00 a.m.
- Arizona and Hawaii do not observe Daylight Saving Time.
During DST, the time difference between zones stays the same. For example, New York is still 3 hours ahead of Los Angeles, even though both move their clocks forward.
Time zone vocabulary
These words help you talk and write about time zones correctly.
- Longitude: The distance east or west of the Prime Meridian, measured in degrees.
- Prime Meridian: The line of 0 degrees longitude that passes through Greenwich, England.
- UTC: Coordinated Universal Time, the world’s standard time.
- Time difference: The number of hours between two time zones.
- Daylight Saving Time: Moving clocks forward one hour in spring and back in fall.
Using these terms correctly will help you explain your thinking in math and social studies.
Common Core alignment: CCSS.MATH.CONTENT.4.MD.A.1 – Know relative sizes of measurement units within one system of units. CCSS.MATH.CONTENT.4.MD.A.2 – Use the four operations to solve word problems involving distances, intervals of time, and conversions. CCSS.ELA-LITERACY.RI.4.3 – Explain events, procedures, ideas, or concepts in a historical, scientific, or technical text.
Notes for teachers
This resource is designed to provide clear explanations, examples, and practice for learners of different needs and levels of support. It may be used for whole-class instruction, guided practice, independent learning, review, homework, or additional reinforcement at home.