Using Month Only in DateTime

I only want to change the MonthlyCalender Month to a variable(May) but for some reason i have to enter the full date which breaks my code, as i search for the month in the variable and then match it so it then excutes the code when it finds the string, below is the code;

if (textDueDate.Contains("May"))             {                 DateTime dt = new DateTime();                 clndDueDate.SetDate(dt.AddMonths(05));                 clndDueDate.Update();                 MessageBox.Show("Inside May");             }             else             {             }

This does nothing, So do i use SetDate property in a different way... since i added the month on the Monthly calender.

MessageBox is just my debugger :P

Maybe substring?

DateTime dt = new DateTime(2001, 01, 01); //Works with full date DateTime dt = new DateTime(null, 01, null); //Not working ;(

Thanks for reading