Longitudes are measured from the prime meridian, which is zero line for longitudes. On Earth, the prime meridian goes north and south through Greenwich, England. It is sometimes called the Greenwich meridian.
Time zones: every country decides on its own what time zone to use. Some countries use more than one. Most counties pick a time zone so that 12:00 noon is when the sun is highest in the sky, most of the time. Also, they define their time zone as some number of hours before or after the standard time at Greenwich. A few countries add a half-hour, and one adds 45 minutes.
There are 40 time zone in use throughout the world. If you look at the time zone map (see the link), you might notice that the "main" time zones are the one-hour ones. There are 24 of these around the world. Also, there are 360 degrees of longitude around the world. So each of the "main" time zones is 15 degrees wide ***on average***. 360 divided by 24 is 15.
But there are 16 time zone that are not "main" time zones. These are the half-hour and 45 minute time zones.