Try this:
string val = "23.12.1992 00:00:00";
// Parse exactly from your input string to the native date format.
DateTime dt = DateTime.ParseExact(val, "dd.M.yyyy hh:mm:ss", null);
// Part to SqlDateTime then
System.Data.SqlTypes.SqlDateTime dtSql = System.Data.SqlTypes.SqlDateTime.Parse(dt.ToString("yyyy/MM/dd"));
This could be done in one statement, but just separated for illustration.