Standard Time Zones
You can understand how a general foul-up would result if all people set
their watch on their own LMT. As you know, local mean time (LMT)
always differs in different longitudes. In a large city, for example, a
difference of about 9 seconds (9s) LMT occurs between one end of the
city and the other end. If you set your watch on your LMT you would
have to change it every time you went a few blocks on a street running
east and west.
To eliminate this difficulty, standard time zones have been established
within which all clocks are set to the same time, zone time (ZT). A
difference of 1 hour (lh) takes place between one time zone and the next.
Because 1h is 15°, you can see that each time zone comprises 15° of
longitude. Thus resulting in 24 time zones, and 25 time zone
Figure 5-3. Time zones.
Local mean time along each standard time meridian is zone time for the
entire time zone. Look at figure 5-3. In zone 0, time is exactly the same
within 7 1/2° either side of the standard meridian. Zone time in
navigation is abbreviated ZT.
Daylight savings time is simple zone time set ahead 1 hour to extend the
time of daylight.