The concept of time zones was developed in 1884 at the International Meridian Conference in Washington.
After this conference, the world was broken down into 24 separate time zones. The objective of time zones is to standardize time measurement around the world and all locations within the same time zone have the same time.
The first one, also known as the Greenwich Meridian is crossed by the prime meridian (0°).
A time zone function on a watch is able to indicate several time zones concurrently.