Why can't a Variant contain a TDateTime before 100 CE?

Question

Consider the following code:

procedure Test;
  function d1: Variant;
  var
    DDt: TDateTime;
  begin
    DDt := EncodeDate(100,1,1);
    Result := DDt;
  end;
  function d2: Variant;
  var
    DDt: TDateTime;
  begin
    DDt := EncodeDate(99,12,31);
    Result := DDt;
  end;
  procedure Writedate(V: Variant);
  begin
    Writeln(string(V));
  end;
var
  V: Variant;
begin
  V := d1;
  Writedate(V);
  V := d2;
  Writedate(V);
end;

The first call to Writedate will succeed, and the output will be '01-01-0100'. The second call, however, will fail with an 'invalid argument' failure. Inspecting the code, you can see the Variant of the 99-12-31 date has a EVariantInvalidArgError error.

However, if I call FormatDateTime('c', TDateTime(V)) on either TDateTime, they will both succeed. In fact, at any point when the Variant contains a TDateTime, whose date is before 100 CE, the IDE will display a EVariantInvalidArgError when inspecting its value.

It seems odd that the Variant cannot handle the pre-100 CE date, when TDateTime can. Is this a bug in Delphi? I find it being right between 99 and 100 CE to be a bit suspicious.

Answer 1

Variant can contain any date value, as your code demonstrates (assignment V := d2; produces no error).

The error is raised during the conversion to string which the compiler delegates to the OS on Windows platforms. This fails because OLE Automation specifies midnight, 1 January 0100 as the minimum valid OLE Automation date value.