<Tr> <Td> </Td> <Td> 2h ± 30m ahead </Td> </Tr> <Tr> <Td> </Td> <Td> 3h ± 30m ahead </Td> </Tr> <P> Ideal time zones, such as nautical time zones, are based on the mean solar time of a particular meridian located in the middle of that zone with boundaries located 7.5 degrees east and west of the meridian . In practice, zone boundaries are often drawn much farther to the west with often irregular boundaries, and some locations base their time on meridians located far to the east . </P> <P> For example, even though the Prime Meridian (0 °) passes through Spain and France, they use the mean solar time of 15 degrees east (Central European Time) rather than 0 degrees (Greenwich Mean Time). France previously used GMT, but was switched to CET (Central European Time) during the German occupation of the country during World War II and did not switch back after the war . Similarly, prior to World War II, the Netherlands observed "Amsterdam Time", which was twenty minutes ahead of Greenwich Mean Time . They were obliged to follow German time during the war, and kept it thereafter . In the mid 1970s the Netherlands, as with other European states, began observing daylight saving (summer) time . </P>

What is the line in the middle of a time zone that in theory determines the time of that zone