Option 1
You can subtract two DateTime objects which returns a TimeSpan object. Here is an example:

//To get the amount of days between two dates.
DateTime date1 = new DateTime(2007,1,1);
DateTime date2 = DateTime.Today;
int daysDiff = ((TimeSpan) (date2 - date1)).Days;

This is possible via operator overloading. Here are the MSDN docs on addition and subtraction.

Option 2
Write you own DateDiff function. Of course somebody already has. Just copy and paste.

Option 3
For VB fans, you can reference the Microsoft.VisualBasic dll and access the VB DateDiff function directly. See the example on this page.