Date Validation using C#

Hi,
I am new with .NET coding. I would like some help in validating a Date. I already have the Calendar extender on my code. What I need help on is validating the date. The scenario goes something like this:

If selected date < systemdate then
message = "registration has expired"
else if (selected date > systemdate and selected date <= systemdate +45) then
message = "additional fees will be incurred"
else if (selected date > systemdate+45 and selected date < systemdate + 90) then
message = "You will be on a waiting list"
else
message = "Application can not be processed"

Help!

Thank you,
Ed