First you need to get it into a datetime object. The most common standards work via:
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.
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