How difficult would it be to make a simple Hebrew/Jewish Calendar covering the last 25 years using paper and pencils (including days of the week and corresponding Gregorian dates), with no access to the Internet, but knowing the current Hebrew and Gregorian date the day you began?
To make a simple, purely Gregorian calendar covering the last 25 years this way is a labourious task and takes about two or three evenings, but would be possible for any average person, provided they remembered when the last leap year was.
However, I came accross an article about Dr. Alan Rosen's The Holocaust's Jewish Calendars: Keeping Time Sacred, Making Time Holy, which discussed the illicit calendars made in Auschwitz. Obviously, these were made under unimagiably difficult conditions (AND they included the weekly Torah portions and holidays), but I was struck by the fact that the article mentioned one of the makers had studied under a teacher who "rigorously taught his young charges the rules for composing Jewish calendars". Furthermore, according to timeanddate.com, the Hebrew/Jewish Calendar features "a body of complex regulations, exceptions, and mathematical rules".
If you didn't include the holidays and Torah portions, how easy would it be for the average person to make such a calendar, and how long would it take?