MaskedTextBox What is that a wrong setting or bug?

I trying to learn SQL 2008 Express as database and Visual 2010 C# as programing language I discover problem with "masked textbox" I used it for "Birthday", "Phone", "Postal Code". The "Phone" works fine but  input of "Birthday",  if put in for instance, type date  which starts with 0 like this format  01/01/2012 it's changed  to 11/20/12_1, The "Postal Code" field saved in database like this format 90833-. I mean with dash in the end. What is that a wrong setting or bug?FD
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