1. Brief description

• The so-called Julian calendar was started in 45 BC, before which the calendar was very imprecise. In the Julian system, leap years were supposed to be every four years, but initially they were inadvertently kept every three years. The Emperor Augustus realised the mistake, and as a result the leap years -9, -5 and -1 (or -5, -1 and +4) were not observed, after which the situation returned to normal.
• The year zero (0) does not exist! After 31.12.-1 comes 1.1.1 (the minus sign represents years BC).
• The so-called Gregorian calendar was started in 1582, when it was discovered that the simple "four-year" rule of the leap day was inaccurate in the long run (the correct length of the year is 365.2422 days). Pope Gregory VIII wanted to correct an error that had already accumulated (since 45 BC), and so after Thursday 4 October 1582 came Friday 15 October 1582, which marked the beginning of the new so-called Gregorian calendar, which we still use today.
• The above Gregorian transition took place at different times and in different ways (even gradually) in different countries.Thus, there was a gradual series of transitions, one after the other, until the 1900s. For a more detailed list of countries and times, see the more extensive description in the link below, as well as Wikipedia.
• The JD number (Julian Day number) is a way of giving all days a sequential number (starting from 1 January 4713 BC). This has nothing to do with the Julian calendar, although the names are similar.
• The Jewish observance of Passover was at first an imprecise calculation, based partly on tables. It was not until 325 that a more uniform method of calculating the time of Passover was adopted in the world, instead of tables. The new formula used in this programme.
• The celebration of Orthodox Easter began in 1054. It still follows the Julian calendar (without the Gregorian calendar correction).
• Week numbering. The international standard ISO-8601 defines a new week as starting on Monday. The same standard also defines how the weeks of the year should be numbered. When a week is divided into two different years, it is numbered according to the year in which most days of that week fall. In general, there are 52 weeks in a year, but those years that start on a Thursday, and on the other hand, leap years that start on a Wednesday, contain 53 weeks. Note that this international day and week numbering system is not followed in the United States, for example.
• The moon phase calculation has an accuracy of better than 0.5 days, which is good enough for tracking the dates given in, for example, Part IV of The Urantia Book. The lunar phase repeats at intervals of 29 days 12 h 44 min and 2.82 seconds.
• Grey text in the results means that there is some uncertainty in the answer, or the result has been extrapolated outside its actual range (for example: the day of the week 1.1.4713 BC is Monday).

2. Else

• Technically, the program accepts all months as 31 days. But it is up to the user to interpret any "extra day" for the correct month. For example, entering date 31 September (which does not exist) means the same as 1 October.
• The calculation formulae have been based on the best available scientific data, almost all of which can be found on the below-mentioned website, its web links or other known sources of information.

3. Broad description

• The formulas are based on data from this calendar website.