The following C# code works perfectly.

OleDbCommand cmd = new OleDbCommand();
cmd.Parameters.Add("@login_timestamp", OleDbType.DBTimeStamp).Value = DateTime.Now.ToString();
cmd.CommandText = "insert into myTable values (?)";

However when converting the same code to C++ you cannot just use OleDbType::DBTimeStamp as this results in an error when inserting the data, instead you have to use OleDbType::Date so the code looks like this.

OleDbCommand ^cmd;
cmd=gcnew OleDbCommand();
cmd->Parameters->Add("@login_timestamp", OleDbType::Date)->Value = DateTime::Now;
cmd->CommandText = "insert into myTable values (?)";