Today, civil time is based on Coordinated Universal Time (UTC). This 24-hour time standard is maintained by combining exceedingly precise atomic clocks with the rotation of the Earth. The rotation of the Earth is a very slow process - once every 24 hours - which allows for an accurate definition of time.
At the most basic level, gravity keeps time for the world. All objects in space fall toward Earth, including planets and stars. The closer an object is to Earth, the more gravity it experiences. Over time, this constant force of gravity slows down Earth's rotation and brings all of its life forms around to sleep at about midnight. When they wake up the next day, everything has changed. It is now morning everywhere on Earth!
But there are many other factors that influence how fast or slow Earth rotates. For example, if the moon were not present in our orbit, then Earth would rotate much faster than it does today. Also, if a large mass such as a mountain range was to collapse, then a lot of energy would be released and Earth's rotation would slow down even more. Finally, some parts of Earth's surface experience more solar radiation than others. The poles are always moving away from the Sun, while the equator is always getting hotter from all of the sunlight that falls on it.
Because Universal Time is governed by the Earth's rotation, which wanders away from more exact atomic-frequency standards, an adjustment (known as a leap second) to this atomic time is required because "broadcast time" stays largely synchronised with solar time (as of 2019). The last such addition was in 2012. Before that, in 2001, 2003 and 2005, respectively.
The International Earth Rotation Service (IERS) is responsible for maintaining close ties with many national observatories around the world that provide data on global positioning system (GPS) satellites. IERS also receives satellite data directly from GPS manufacturers who want to have it adjusted into a form that can be used by IERS scientists.
GPS satellites transmit information about their position and orientation using radio waves. Because they are orbiting the planet at approximately 11,000 miles (17,500 km) above the surface, each one provides a different view of part of the globe. By comparing the signals from several satellites, it is possible to calculate the location of every GPS receiver on Earth.
In order for this calculation to be accurate, all components of the system must operate consistently and correctly. IERS monitors these activities closely, ensuring that any irregularities are reported to national authorities so that they can make any necessary adjustments to their clocks.
Time keeps changing.
So, in some ways, time is the same for all countries throughout the world since all times have an official UTC (Universal Time Coordinated) equivalent, even if they use a different local time (offset from the UTC). You may hear that the official time in certain regions is +6 or -7 or anything along those lines. These are called daylight savings time offsets and they vary by region between the months of March through October.
When people talk about "Eastern Standard Time" or "Central Standard Time", they are referring to places that observe Eastern Daylight Time or Central Daylight Time respectively. The term "Standard Time" itself refers to an hour ahead of UTC; thus, every place that observes Standard Time is also on UTC+00:00 but varies which hour gets added/subtracted from it each day.
Time zones were first introduced by the United States as part to their World War II war effort. Before then, most countries across the globe had individual laws regarding when clocks could be turned back and forth by the mayor or local government during winter months when sunlight was needed for warmth. But as part of its war effort, the United States decided to have one single rule for the whole country: at 2am on April 14th, 1945, all commercial clocks would move forward one hour starting with Washington, D.C.
During World War I, the British Empire had adopted 11:30pm as its own curfew time for young men under military age.