The Eastern Time Zone contains 17 states in the eastern part of the Continental United States and is shared by parts of Canada and three countries in South America. These places use Eastern Standard Time (EST) when observing standard time (autumn/winter) - which is 5 hours behind Coordinated Universal Time (UTC−05) - and Eastern Daylight Time (EDT) when observing daylight saving time (spring/summer) - which is 4 hours behind (UTC−04). On the second Sunday in March, at 2:00 am EST, clocks are advanced to 3:00 am EDT leaving a one hour gap. On the first Sunday in November, at 2:00 am EDT, clocks are moved back to 1:00 am EST.

The 1966 Uniform Time Act in the United States meant that EDT was instituted on the last Sunday in April, starting in 1966. EST would be re-instituted on the last Sunday in October. The act was amended to make the first Sunday in April the beginning of EDT as of 1987. The Energy Policy Act of 2005 extended daylight saving time in the United States beginning in 2007, so that the local time changes at 2:00 am EST to 3:00 am EDT on the second Sunday in March and returns at 2:00 am EDT to 1:00 am EST on the first Sunday in November. In Canada, the time changes as it does in the United States.

A time zone is a region on Earth that has a uniform standard time for legal, commercial, and social purposes. It is convenient for areas in close commercial or other communication to keep the same time, so time zones tend to follow the boundaries of countries and their subdivisions.

Most of the 40 time zones on land are offset from Coordinated Universal Time (UTC) by a whole number of hours (UTC−12 to UTC+14), but a few are offset by 30 or 45 minutes. Some higher latitude countries use daylight saving time for part of the year, typically by changing clocks by an hour. Many land time zones are skewed toward the west of the corresponding nautical time zones. This also creates a permanent daylight saving time effect.

Before the invention of clocks, people marked the time of day with apparent solar time (or "true" solar time) – for example, the time on a sundial – which was typically different for every settlement.

When well-regulated mechanical clocks became widespread in the early 19th century,^{[citation needed]} each city began to use some local mean solar time. Apparent and mean solar time can differ by up to around 15 minutes (as described by the equation of time) due to the non-circular shape of the Earth's orbit around the sun. Mean solar time has days of equal length, and the difference between the two averages to zero after a year.