Creates a date object denoting the given calendar date.
In this class, BCE years are counted astronomically.
In those classes, those are so-called “ajd” and “jd”.
The modified Julian day number is in elapsed days since midnight (Coordinated Universal Time) on November 17, 1858 CE (in the Gregorian calendar).
The offset is relative adjustment, which determines decoded local time with the day count.
The day of calendar reform denotes the start day of the new style.
A Date object is created with ::new, ::jd, ::ordinal, ::commercial, ::parse, ::strptime, ::today, Time#to_date, etc.