The isDayExcluded method doesn't check the base calendar. and because of that the getNextIncludedTime function can return an invalid date, according to the base calendar.

If you just add

        // Check baseCalendar first
        if (! super.isTimeIncluded(day.getTime().getTime())) {
         return true;
        }

to the top of the isDayExcluded method (just belof the check if (day == null), it should be fine, if I'm not mistaken.