Let me show you how that works with Example 2 — Page 408, Lesson 362

We're now into the heart of time problems in navigation, and I want to walk you through the worked examples, because they show you the exact method you'll use in the exam. The key principle throughout is that we always convert through UTC — that's our common reference. Let me show you how that works with Example 2. We're asked to find the LMT in Vancouver, British Columbia, at longitude 123 degrees West, when the Standard Time there is 0008 hours on the 5th of March. So we start with Standard Time Vancouver: 5th, 00 hours, 08 minutes. The Standard Time Difference for Vancouver comes from List 3 in the Air Almanac, and it's plus 8 hours. Adding that gives us UTC: 5th, 08 hours, 08 minutes. Now we convert from UTC to LMT. Vancouver is at 123 degrees West, and the arc/time conversion for longitude West is minus 8 hours 12 minutes — that's the "UTC BEST" step, because going west we subtract. So we subtract 8 hours 12 minutes from 08:08, and we get LMT Vancouver: 4th, 23 hours, 56 minutes. Notice the date went back to the 4th because we subtracted enough to cross midnight. Now Example 3 — the reverse direction. We're finding Standard Time in Fairbanks, Alaska, at 148 degrees West, given the LMT is 0614 on the 18th of February. We start with LMT Fairbanks: 18th, 06 hours, 14 minutes. To get to UTC, we add the arc/time for longitude West, which is plus 9 hours 52 minutes — that's the "UTC BEST" step again. That gives us UTC: 18th, 16 hours, 06 minutes. Then we apply the Standard Time Difference for Fairbanks from List 3, which is minus 9 hours. Subtracting that gives us Standard Time Fairbanks: 18th, 07 hours, 06 minutes. Example 4 is the one that really tests you, because it involves two different places. We need the Standard Time in Georgetown, Guyana, at 08 North, 060 West, when the LMT in Sydney, New South Wales, at longitude 151 degrees 20 minutes East, is 1116 hours on the 21st of January. So we start with LMT Sydney: 21st, 11 hours, 16 minutes. Sydney is East, so we subtract the arc/time for longitude East — that's minus 10 hours 05 minutes, the "UTC LEAST" step. That gives us UTC: 21st, 01 hour, 11 minutes. Then we apply the Standard Time Difference for Guyana from List 3, which is minus 3 hours. Subtracting that gives us Standard Time Georgetown: 20th, 22 hours, 11 minutes. Again, the date went back to the 20th. Now, Examples 5 and 6 are the ones that involve the International Date Line, and this is where the summary at the end becomes critical. Let me read you that summary, because it ties everything together. LMT is based on the movement of the mean Sun, which travels through 15 degrees of longitude per hour. UTC is simply the LMT on the Greenwich Meridian. The difference in LMT at two places is found by finding the difference in their longitudes and converting it into time at the rate of 15 degrees per hour — except where the shorter longitudinal arc spans the date line, in which case you work through UTC. And you must be prepared to calculate the time equivalent of longitude without using the Air Almanac. Zone Time is in the EASA syllabus, so be prepared to convert longitude into zone number and to convert between Zone Time and UTC. Standard Time is used to prevent the extreme inconvenience of using LMT on clocks. The Standard Time Differences for all countries are found in Lists 1, 2 and 3 in the Air Almanac. Conversion of LMT to Standard Time, or vice versa, is best done through UTC. And LMT problems involving an aircraft crossing the International Date Line are also best done through UTC. Now let me show you why that matters with Example 5. An aircraft left position A at longitude 164 West on a westerly heading at 2200 hours LMT on the 3rd of May. We need the LMT and local date of arrival at position B at longitude 173 East, with a flight time of 6 hours. We start with Depart A: 3rd, 22 hours, 00 minutes. Position A is West, so we add the arc/time for longitude West — plus 10 hours 56 minutes, the "UTC BEST" step. That gives us Depart A in UTC: 4th, 08 hours, 56 minutes. Then we add the flight time of 6 hours, giving us Arrive B in UTC: 4th, 14 hours, 56 minutes. Now we convert to LMT at B. Position B is East, so we add the arc/time for longitude East — plus 11 hours 32 minutes. That gives us Arrive B: 5th, 02 hours, 28 minutes. Notice the date jumped forward to the 5th — that's the date line effect. Example 6 is the mirror image. An aircraft left position X at longitude 175 East on an easterly heading at 0100 hours LMT on the 18th of November. We need arrival at position Y at longitude 150 West, flight time 4 hours. We start with Depart X: 18th, 01 hour, 00 minutes. Position X is East, so we subtract the arc/time for longitude East — minus 11 hours 40 minutes, the "UTC LEAST" step. That gives us Depart X in UTC: 17th, 13 hours, 20 minutes. Notice the date went back to the 17th. Add the flight time of 4 hours, giving Arrive Y in UTC: 17th, 17 hours, 20 minutes. Now convert to LMT at Y. Position Y is West, so we subtract the arc/time for longitude West — minus 10 hours 00 minutes. That gives us Arrive Y: 17th, 07 hours, 20 minutes. The crucial lesson from these two examples is this: when the shorter arc between two longitudes crosses the date line, you must not try to convert directly between the two LMTs. You always go through UTC — convert the departure LMT to UTC, add the flight time, then convert from UTC to the arrival LMT. That's the only safe way to handle the date change correctly. And remember, the arc/time conversions are always at the rate of 15 degrees per hour, and you must be able to compute those time equivalents yourself, without the Air Almanac.