There is a solution here: http://stackoverflow.com/questions/1617049/calculate-the-number-of-business-days-between-two-dates
] which will need a small change to include Saturdays - it assumes a 5 day working week, where you have a six day.
But please, you don't need to convert like that!
Don't play with text: look at using DateTime.TryParseExact
] instead - it lets you specify exactly what the input string should look like, and convert it in a single operation (or report an error if the user entered a bad date).