Looks like your DateSeparator of your CurrentCulture is - and that's why / character replace itself to it.
"/" custom format specifier has a special meaning as replace me with current culture or supplied culture date separator.
You have a few options, you either escape it with single quotes (or \/ in a verbatim string literal) or use a culture that has / as a DateSeparator like InvariantCulture.
string s = DateTime.Now.ToString("M'/'d'/'yyyy");
string s = DateTime.Now.ToString(@"M\/d\/yyyy");
string s = DateTime.Now.ToString("M/d/yyyy", CultureInfo.InvariantCulture);