datetimepicker值为类型 DateTime 它有属性日，月，年。
要访问它们：

dateTimePicker1.Value.Day;
dateTimePicker1.Value.Month;
dateTimePicker1.Value.Year;

这些属性是只读的。要修改它，请使用以下方法：
edit（thx to@richard）：adddays、addmonths、addyears方法返回新的datetime值，您必须将该值赋回datetimepicker.value

dateTimePicker1.Value = dateTimePicker1.Value.AddDays(_days);
dateTimePicker1.Value = dateTimePicker1.Value.AddMonths(_months);
dateTimePicker1.Value = dateTimePicker1.Value.AddYears(_years);