A perpetual calendar is a calendar that will give you the day of the week at any date in any year. So it would tell you for example that January the 11th in 2023 is a Wednesday. This works because the only part of the day and date never repeats itself: the year. Days, weeks, months all keep repeating themselves throughout their cycle. This can be used to fit a calendar into a watch that will continue to proceed through these cycles and never run out. Plus it gives you the day of the week.

Since the going theory stipulates that the earth is not flat and the sun is not the sun-god Ra in his golden chariot, but that we actually live on a ball of dirt flying around a star, it is obvious that the daytime will never be the same on any two points of different longitude at any given time. Now if you were going to be precise, then you are actually in a different time of day than your neighbours to the east and west of you. But that would be splitting hairs, or would it? Anyways, important people decided a long time ago that for practicalities sake, they would divide the globe into 24 time zones that are one hour apart. Of course one had to find a compromise between geographical position and national borders in order for ordinary life to function. Since the industrial revolution, people adhering to the same principles of time measurement have become essential to the workings of our society. But you will find that every so often a time zone will shift in order to make life easier or more profitable in one country or region. For example the UK is considering not of moving to a different time zone, that would be rubbish of course, since they invented it, but of adopting “double summer time” in one year and then only going back one hour at the end of it. This would effectively put them in the same time zone as the rest of Europe.