My go-tos for DateTime Input and Output:
http://www.dotnetperls.com/datetime-parse for input (parsing)
http://www.csharp-examples.net/string-format-datetime/ for output (formatting)
string dateString = "01 01 1992";
string format = "MM dd yyyy";
DateTime dateTime = DateTime.ParseExact(dateString, format, CultureInfo.InvariantCulture);
Edit since his edit makes my above answer irrelevant (but will leave there for reference):
From what you're saying, you want to output today's date in a dynamically-defined format?
So if I want to see month, date, year, I say "MM dd YY" and you return it to me?
If so:
DateTime dt = DateTime.Today; // or initialize it as before, with the parsing (but just a regular DateTime dt = DateTime.Parse() or something quite similar)
Then
String formatString = "MM dd YY";
String.Format("{0:"+ formatString+"}", dt);
Your question is still quite unclear, though.