Convert dd/mm/yy to dd-mm-yyyy

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)){
   // Convert is ok
}
else{
   // Could not convert
   // The string contains a invalid date or invalid dateformat
}

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[^]

You have a number of answers, but your root problem is different. It try to work with string representation of data, instead of data itself. This is wrong. Work with data, in this case, System.DateTime data. You don't usually need to convert one string into another. You need to work with data all the time until you need to present it on screen. In that case, use one of the System.DateTime.ToString functions. Yes, as simple as that.

—SA

Date dt="9/1/2001";
String s=dt.ToString("dd-MM-yyyy");