Frequently Asked Questions About Time Zones

A timezone is a region of the Earth that observes a uniform standard time. Time zones are defined by their offset from Coordinated Universal Time (UTC). As you travel east from the Prime Meridian in Greenwich, England, the time increases by one hour for each 15 degrees of longitude.

There are 24 primary time zones, each separated by one hour. However, some regions use half-hour or quarter-hour offsets (like India at UTC+5:30 and Nepal at UTC+5:45), bringing the total number of unique time offsets to over 37.

UTC (Coordinated Universal Time) is the primary time standard by which the world regulates clocks. GMT (Greenwich Mean Time) is the mean solar time at the Royal Observatory in Greenwich, London. For most practical purposes, UTC and GMT are the same, though UTC is the modern standard used internationally.

Daylight saving time (DST) is the practice of advancing clocks by one hour during warmer months so that darkness falls at a later clock time. In the United States and Canada, clocks spring forward one hour on the second Sunday of March and fall back on the first Sunday of November. Not all countries observe DST.

Time zones were created to standardize timekeeping across different regions. Before time zones, each city set its own local time based on the position of the sun. The adoption of standardized time zones began in the late 1800s, largely driven by the expansion of railway networks which needed consistent scheduling.

To convert time between two zones, find the UTC offset of each zone and calculate the difference. For example, New York (UTC-5) to London (UTC+0) is a 5-hour difference — when it is 12:00 PM in New York, it is 5:00 PM in London. You can use the compare feature on Timebyplace.com to easily convert times between any two cities.

The International Date Line (IDL) is an imaginary line running roughly along the 180th meridian in the Pacific Ocean. When you cross the line traveling westward, you add a day; when crossing eastward, you subtract a day. The line zigzags to avoid dividing countries and island groups into different dates.

Most countries near the equator do not observe DST because the amount of daylight does not vary significantly throughout the year. Notable countries that do not use DST include Japan, China, India, South Korea, and most of Africa and Southeast Asia. In the United States, Arizona and Hawaii do not observe DST.