Rebase to daycount since proleptic year 1.
Add count of days for cycles.
Change yearsUntil to deal with inserted leap months.
Implement ofEpochDay for positive years.
Make century-year work for negatives, add missing sixCycle value.
Add ofEpochDay for negative years.
Correct isLeapYear calculation (400 _not_ a leap year)
Correct for no offset in year.
Divide by days in month to get month of year.
Correct test data.
Correct ofEpochDay errors.