If you look at a World map (such as the one in front of me here), you'll see that the World has a series of 18 major "vertical" lines on it (lines of latitude). (OK, so each of those is subdivided and subdivided but we're only concerned timewise with the 18 major lines).

There are 18 of them and that means that each represents 20º of longitude which in turn means that each has therefore 80 minutes of time between them.

Just where do you start "counting" from? = You start at zero and you're either East or West of that line. Where does that line pass through? Greenwich, that's where.

In the World, there are no end of agitators who would dearly love to move that line someplace else and do away with what is known Worldwide as Greenwick Mean Time.

Today, I don't really see the need for BST or Daylight Saving Time and I would like to see the World stick to GMT as the standard time upon which each time zone determines whether they are GMT + or GMT - and be done with clocks on or clocks back.