DateTime format question

Ok, I have created a custom CreateUserWizard with additional steps that save to the profile in the UserCreated event.  This line is throwing an error that 9/4/2008 : String was not recognized as a valid DateTime.  Also what would be the best way to handle null values?  Condition on text for the text box?  I am tired been working on this thing all day and now this pops up.  Any help would be appreciated.

p.BirthDate = DateTime.ParseExact(txtBirthDate2.Text, "MM/dd/yyyy", System.Globalization.CultureInfo.InvariantCulture);

Thank you!

Jason
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