Move leap-week offsetting to AccountingPeriod.
Swap out method for cached instance (differences tested in builder).
Correct which variable checked for flag value(>_<).
Add test, correction for elapsedWeek -> month issue.
Correct test data.
Implement ofEpochDay, minor bug fixes.
Add check for lower bound on day-of-month.
More test-data fixes.
Override lengthOfYear.
Fix more test data (I need to work on multiples of 7).