The Earth rotates on its axis and every 24 hours, it completes one full rotation. These 24 hours are also shown on our clocks. The majority of the time is split into 12 a.m. and 12 p.m. hours. Greenwich Mean Time, also known as Universal Time, is used when we choose to use a common time that is the same all over the world. On a 24-hour clock, this is calculated.
There are 24 time zones on the earth. The time is the same on all clocks in the same time zone. Each time zone is one hour ahead of the next east zone and one hour behind the next west zone. On the equinox, each hourly movement of the sun approximately represents the transition from one time zone to the next.
The earth rotates 360° in about 24 hours.
360 degrees/ 24 hours
= 15 degrees per hour.
∴ Each time zone covers 15 degrees of longitude.