No, TDateTime is absolutely year 2000 complient. The problem is that you are setting the field as a string and something is being lost in the process. What you need to do is read in the TDateTime as a TDateTime by using a TDateField or a TDateTimeField. Instead of this:

AnsiString as = DataSet->FieldByName ("FieldName")->AsString;
TDateTime dt (as, TDateTime::Date);
short int y, m, d;
dt.DecodeDate (&y, &d, &y);
y = 200 ;
TDateTime dt2 (y, m, d);
DateSet->Edit ();
DataSet->FieldByName ("FieldName")->AsString = dt2.DateString();
Do this:
TDateField *df = (TDateField*)DataSet->FieldByName ("FieldName");
TDateTime dt = df->AsDateTime;
short int y, m, d;
dt.DecodeDate (&y, &d, &y);
y = 200 ;
TDateTime dt2 (y, m, d);
DateSet->Edit ();
df->AsDateTime = dt2;