Guide Timezones
The Issue
usually we use local time, which is correlated to the sun's position. but what to do, if one person is located in UK and another person is located in Austrialia, how can they arrange a meeting?
The Soution
In the past GMT was the clocktime, which never changed. Nowadays UTC is used, which is (the more the less) an equivalent/synonym for GMT, ZULU is another synonym
so if you travel around the world, you need to change your clock's time every 15° longitude. Timezones reflect this.
Every 15°, if east to Greenwich, add 1 hour, if west substract 1 hour.
To actually setting your clock to correct time, you need to know GMT/UTC time and add the timezone offset to it. this gives you your local standard time (LST) (GMT + timezone offset = LST)
For saving energy summertime was invented in War I. so this offset needs to be added to the LST to giving you the time which then is called local civil time (LCT) (LST + summertime offset = LCT)