During Daylight Saving Time (DST) the time zone name and time changes. The words “daylight” or “summer” are then usually included in the name, and the local time is usually set forward 1 hour.
For example, California uses Pacific Daylight Time (PDT) during the DST period with a UTC offset of UTC-7, but Pacific Standard Time (PST) with an offset of UTC-8 during the rest of the year.
Not the Same as Local Time
The term time zone is often confused with local time. For instance, during DST, it is common to say “California and Arizona are now in the same time zone.” However, the correct thing to say would be: “California and Arizona now have the same local time.”
The reason it may be confusing is that California's local time during DST is UTC-7, but the standard time in California is UTC-8. However, Arizona's local time is always UTC-7, because there's no DST in Arizona, and they remain on standard time all year.
Time Zone Borders Vary
Theoretically, each 1-hour time zone is 15 degrees wide, indicating a 1-hour difference in mean solar time. This can be seen as the white and gray stripes on our Time Zone Map and in the image above.
The actual borders on a time zone map have been drawn to correspond with both internal and international borders, and rarely match up exactly with the 15-degree time zone borders.
Some geographically large (wide) countries, like India and China, use only 1 time zone, while it would have been natural to expect several, like in the US or Australia.
Defined by UTC Offset
Every place on Earth is measured in terms of its distance east or west of the prime meridian (0°longitude) in Greenwich, London, United Kingdom. This is also the reference point for Coordinated Universal Time (UTC) with 1 hour per 15 degrees longitude.
You have to divide the longitude, in degrees, by 15 to find the appropriate time zone, in hours. For example:
At 150 degrees west (or 150° W) longitude, the time should be 150 degrees divided by 15 degrees = 10 hours behind UTC, or UTC-10.
At 75 degrees east (or 75° E) longitude, the time would be 75 degrees divided by 15 degrees = 5 hours ahead of UTC, or UTC+5.
Below are all the different local times currently in use worldwide.
