Add time Minutes in wp8

I want to add time 5 minutes in TimePicker. Why the  beginTime is not change?
ex. msg3 = 12:20:00

beginTimePicker.Value = DateTime.Parse(msg3.Replace(":00", ""));
DateTime date = (DateTime)beginDatePicker.Value;
DateTime time = (DateTime)beginTimePicker.Value;
time.AddMinutes(5);

DateTime beginTime = date + time.TimeOfDay;