Pick a date from the datetimepicker

I have a car rental application I am writing. I am having some issue. Basically I want a user to pick a date from dateTimePicker2 then dateTimePicker1 I then want to subtract those numbers from eachother and store it in car. I have created a button that will multiple the per day charge then display it in a label but I just having an issue with this part.

carRental = dateTimePicker2 - dateTimePicker1;