[MarcoB]
This solution will perform badly, since for dateTime.Year, dateTime.Month and dateTime.Day is a method and a property get operation called.
[/MarcoB]
look at this:
private string ConvertDatetimeToValid(DateTime dateTime)
{
string datetime = string.Concat(dateTime.Day.ToString(), "/", dateTime.Month.ToString(), "/", dateTime.Year.ToString());
return datetime;
}