From: Frank Reed
Date: 2021 Dec 5, 06:30 -0800
No, you don't need to worry about the International Date Line because you do all the work in terms of Universal Time (equivalent to GMT) and therefore the date is the Greenwich date. You enter the almanac(s) with the Greenwich date in every case.
Mariners at sea do keep a sort of civil time in their logbooks and other official records. This ship time does change the date when a vessel crosses the date line. In the modern world, as I understand it, this ship time ("sort of civil time") is used primarily for setting work schedules aboard ship and also (in some cases) for communicating with local/regional civil authorities. And since it is the logbook and official record-keeping time, it is the time referenced in legal disputes also. Ship time can easily be ignored for celestial navigation, but historically (as recently as 30 years ago) watches and clocks were much rarer, and mariners were taught to do the sometimes annoying conversion from ship time and date to UT time and date. There is still an absurd tradition in shipboard celestial navigation of recording sights in local zone time (the tradition continues in large part because the exams require it). In the modern world where clocks and devices for keeping multiple times are plentiful, this is all quite pointless. Wear a UT watch (or keep a UT date app on your smartphone). Record all your sights in UT, and be done with it.
There are really two International Date Lines. Mariners by custom and international agreement use the 180° meridian of longitude as their date line. You cross it going west, you skip a day, cross it going east you repeat a day. This "Mariners' Date Line", the 180° longitude line, is unambiguous. The other date line is the one commonly found on maps and globes. This "Common Date Line" does not really exist. It is a mapmakers' convention, and all those zigs and zigs and big diversions around some Pacific island nations like Kiribati are inventions of the cartographers. They have no standing in law. It is, however still possible to apply the "Common Date Line" aboard an aircraft or a vessel, maybe for the amusement of passengers. Crossing the Pacific from Oahu to the Cook Islands, you might cross the CDL two, four, or even a higher multiple of two times if you plan a complex course. It will be a multiple of two because you'll be back on the same western hemisphere date standard when you reach your final destination. But of course, since the only thing that counts is the time at the destination, you don't need to make any changes at all. Even in cases where your destination has a different date practice, you could wait until you arrive, or arrive inside their 12-mile limit, to change your date.
Finally, what do you do if you've lost it? In your games crossing the date line (either one), maybe crossing and re-crossing to entertain your passengers, you have become confused, and now you're not sure whether it's December 4 or December 5 by Greenwich date. There's an easy way out if you can see the Moon. No horizon is required, but it's easier at night. Turn to your almanac data, and find the Moon's GHA and Dec for the correct GMT on the two possible dates that you're confused about. Convert the GHA to SHA by subtracting GHA Aries. Then find a start chart (there's one in the almanac) or plot one by hand using the SHA and Dec values of nearby stars in the almanac. Draw in the Moon's position for the two possible dates. Compare against the sky. The Moon's actual position among the stars would be shifted by as much as a degree by parallax in altitude, but you can ignore that here. Only one plotted date will match the Moon's actual position. You have then recovered the Greenwich date. And note, too, that you can apply this in backyard navigation practice. If you've simply talked yourself into confusion over the Greenwich date, and you don't feel like asking the internet, you can get the Greenwich date, as above, by turning to the Moon.