DateTime febDate = new DateTime(2014, 2, 20);
DateTime pickerDate = myDateTimePicker.Value;
TimeSpan tspan = febDate - pickerDate;
int differenceInDays = tspan.Days;
string differenceAsString = differenceInDays.ToString();
If differenceInDays < 0
then multiply it by -1.
Note: In this case it's very easy to get the difference in hours, minutes or seconds as well.
Here's an example of the above code in it's own method:
TimeSpan getDateDifference(DateTime date1, DateTime date2)
{
TimeSpan ts = date1 - date2;
return ts;
}
And when you want to trigger this method:
TimeSpan difference = getDateDifference(new DateTime(2014, 2, 20), dateTimePicker.Value);
//Now you can do what you want with the TimeSpan.
int differenceInDays = difference.Days;
int differenceInHours = difference.Hours;
Console.WriteLine(differenceInDays.ToString());