public Era (int nr, int start, int end)
{
if (nr == 0)
throw new ArgumentException ("Era number shouldn't be zero.");
_nr = nr;
if (start > end)
throw new ArgumentException ("Era should start before end.");
_start = start;
_end = end;
_gregorianYearStart = CCGregorianCalendar.year_from_fixed (_start);
int gregorianYearEnd = CCGregorianCalendar.year_from_fixed (_end);
_maxYear = gregorianYearEnd - _gregorianYearStart + 1;
}