Your real-time culture date separator seems to be -why you get it. You must specify InvariantCulture:
string dateString = dt.toString("MM/dd/yyyy HH:mm", CultureInfo.InvariantCulture);
See: "/" Custom Format Specifier
"/" , , . DateTimeFormatInfo.DateSeparator .
- / \:
string dateString = dt.toString(@"MM\/dd\/yyyy HH\:mm");
, , / " ", ( ) CultureInfo ( InvariantCulture) .