If your take any two days as DateTimes (which teh DateTimePicker will give you as the Value property, you can subtract them to give you a TimeSpan.
This has a Days property which will tell you what you need.
Dim d1 As New DateTime(2011, 1, 1)
Dim d2 As New DateTime(2011, 12, 31)
Dim diff As TimeSpan = d2 - d1
Console.WriteLine(diff.Days)