One thing to note in addition to the other answers - / is a format character itself, representing the local date separator. If you want to make absolutely sure it uses an actual slash, either use the invariant culture (which uses a slash):
string s = dateTime.ToString("dd/MM/yyyy", CultureInfo.InvariantCulture);
or escape the slashes:
string s = dateTime.ToString("dd'/'MM'/'yyyy");