DateTime x = DateTime.Parse(txtDate.Text);
If you expect a freaky format, you still have to know what format it
is:
DateTime x;
DateTime.TryParseExact(txtDate.Text, "YYddd", out x);
Then simply output the data:
string date = x.ToString("MM/dd/yyyy");
But you really need to enforce your formatting using regex,
validators, scout's honor - something.