So, your problem is that you are sending the wrong data type. FormatDateTime returns a String. The last line returns a Date. Have you tried sending a string?
Dim myDate As Date = CDate("1/1/10")
myDate = FormatDateTime(myDate, DateFormat.GeneralDate)
myDate = CDate(myDate)
The result is 1/1/2010
If this is still not what you want, then maybe you could give me your sample code that doesn't work.