Let’s pick this up with the zone time system itself — Page 402, Lesson 354

Let’s pick this up with the zone time system itself. I want you to picture the Earth divided into 25 zones, each spanning 15 degrees of longitude, which equals one hour of time. The only exceptions are the two semi-zones, labelled M and Y, which sit adjacent to the International Date Line. Each full zone is centred 7.5 degrees either side of a central meridian — the first central meridian is the Greenwich Meridian, and then every 15 degrees east and west from there. Each zone is assigned a letter, and the zone time differs from UTC — which is Z time — by whole hours, counting east as plus and west as minus. So Singapore, in zone G, is UTC plus 7 hours. Now, the calculations. You may be asked to find the zone number for a given longitude, or to convert between Zone Time and UTC. To get the zone number, divide the longitude by 15 and round to the nearest whole number, with .5 rounding up. Easterly longitudes get negative zone numbers, from -1 down to -12. Westerly longitudes get positive zone numbers, from +1 up to +12. The zone number is applied to the Zone Time to give UTC. To go the other way, from UTC to Zone Time, you reverse the signs. A handy memory aid: if it’s midday in London, it’s evening in the Far East and morning in the Wild West — or, breakfast at Tiffany’s in New York, lunch in London, tea in India. Mathematically, the rule is: UTC equals Zone Time plus Zone Number. Let me walk you through the examples. Position A is 40S 137E — latitude is irrelevant here. 137 divided by 15 is 9.13 hours, rounded to 9. Longitude east is negative, so the zone number is -9. Position B is 60N 097°30’W — that’s 97.5 degrees west. 97.5 divided by 15 is 6.5, and .5 rounds up, so 7. West is positive, so the zone number is +7. Example 2: at 10S 130E, Zone Time is 0600 on 01 January 00. Zone number is 130 divided by 15, which is 8.66, rounded to -9 because it’s east. UTC equals Zone Time plus Zone Number: 01 0600 plus -9 gives 31 2100 UTC on 31 December 99. Example 3: at 0300 UTC on 17 May, find Zone Time at 20N 155W. Zone number is 10.33, rounded to +10 because it’s west. Rearranging, Zone Time equals UTC minus Zone Number: 17 0300 minus +10 gives 16 1700 on 16 May. Common sense checks out — in Example 2, UTC is less than Zone Time, and in Example 3, Zone Time is less than UTC.