There are two things you'll have to do. First off all convert your string to a DateTime:
string dateString = "9/1/2001";
DateTime dateValue;
if(DateTime.TryParse(dateString, out dateValue)){
}
else{
}
Then if the convert is ok, you can format it the way you need
string formatedDate = dateValue.ToString("dd-MM-yyyy");
List of date format patterns:
http://www.geekzilla.co.uk/View00FF7904-B510-468C-A2C8-F859AA20581F.htm[
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