* Tests are unaffected by this change.
* Zone offset and Gregorian cutover are computed from the DateTime.
* They are still copied in instances variables for ease of compatibility with current code.
* @ajd need to be preserved for now as DateTime only has millisecond precision,
date.rb has arbitrary (although mentioned not expected > ns) (via Rational).
* Joda methods are used for year,week,day,hour,min and second
(but not fractions since it might be imprecise).