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function modifyDate(obj_date, modifier) 
{
	var pattern = /([+-]?[0-9]+)\s*(d|D|w|W|m|M|y|Y)?/g;
	var matches = pattern.exec(modifier);
	while (matches) {
		switch (matches[2] || 'd') {
			case 'd' : case 'D' :
				obj_date.setDate(obj_date.getDate() + parseInt(matches[1],10));
				break;
			case 'w' : case 'W' :
				obj_date.setDate(obj_date.getDate() + parseInt(matches[1],10) * 7);
				break;
			case 'm' : case 'M' :
				obj_date.setMonth(obj_date.getMonth() + parseInt(matches[1],10));
				break;
			case 'y': case 'Y' :
				obj_date.setYear(obj_date.getFullYear() + parseInt(matches[1],10));
				break;
		}
		matches = pattern.exec(modifier);
	}
	return obj_date;
}