title: Delphi: How to determine and empty TDatetime value
body: <p>Seems there is no way to assign <i>NULL</i> (either an "<i>unassigned value</i>" to <code>TDateTime</code> variables.</p>
  
  <p>The only way I've imagined is using something like this:</p>
  
  <pre><code>function isNull(aDate : TDateTime) : boolean;
  const NullDate = 0.0;
  var aNullDate : TDatetime;
      ms : Int64;
  begin
    aNullDate := NullDate;
    ms := MilliSecondsBetween(aDate,aNullDate);
    result := (ms = Int64(0));
  end;
  </code></pre>
  
  <p>Is there anybody out who knows better solution what not overlaps 0 date value?</p>
  
  <p>Are negative <code>TDateTime</code> values dangerous? (As an able resource for previous purpose)</p>