_date1 = _dc.fromString(date1Str);
// setting _date2
ValueExpression<Date> date1Ve, date2Ve;
date1Ve = new ValueSpecification<Date>(_dc, _date1);
date2Ve = new DateSum(date1Ve, unit, interval);
_date2 = date2Ve.eval(null);
// tuple is set to null since we are computing based on constants
}