Arbitrary
Calendar is widely used as a unit of time. It is not synchronized to the
Moon or the Sun. Thus, arbitrary calendar has no astronomical basis. The
best examples of the Arbitrary calendars are the week and the Julian
day. A Natural Calendar has no made up structure and reflects the
observations of the natural phenomena. In any other calendar, dates are
determined by predictions like movement of earth or moon etc. On the
other hand, arbitrary dates are totally meaningless. But these dates can
easily be calculated mathematically. Thus, arbitrary calendar is neither
natural nor meaningful.
Week: Week is that unit of time which is longer than a day but
shorter than a month. In the most popularly used calendars, week is a
period of seven days.
Julian Day: It is a very simple calendar. The calendar date is
just an integer. The Julian Day system was introduced by the astronomers
with a single system of dates to make it easier while working with
different calendars. This is useful for reference, computations and
conversions. The Julian Date is the number of days that have elapsed
since noon Greenwich Mean Time. The current value is 2454086.58889. The
largest integer smaller than this value gives the Julian Day number.
