How to calculate the time zone?

The earth is divided into 24 time zones, 12 in the east and 12 in the west. With Greenwich Observatory in London as the zero time zone, the east 1 zone to the east 12 zone is in the east, and the west 1 zone to the west 12 zone is in the west. The eastern district is earlier than the western district. Longitude is a time zone every 15 degrees. For example, Beijing is located on the meridian of east longitude 120 degrees, belonging to the East Eighth District.

That is, the required time zone = divided by 15 degrees (the remainder is rounded to 4).