[ContraVeneno]

Y la implementación de la función YearSpan es esta:

Código Delphi [-]
Var
  { average over a 4 year span }
  ApproxDaysPerMonth: Double = 30.4375;
  ApproxDaysPerYear: Double  = 365.25;

  { The above are the average days per month/year over a normal 4 year period. }
  { We use these approximations because they are more accurate for the next }
  {  century or so.  After that you may want to switch over to these 400 year }
  {  approximations... }
  {    ApproxDaysPerMonth = 30.436875 }
  {    ApproxDaysPerYear  = 365.2425 }

.....
function YearSpan(const ANow, AThen: TDateTime): Double;
begin
  Result := DaySpan(ANow, AThen) / ApproxDaysPerYear;
end;
...
function DaySpan(const ANow, AThen: TDateTime): Double;
begin
  Result := SpanOfNowAndThen(ANow, AThen);
end;

function SpanOfNowAndThen(const ANow, AThen: TDateTime): TDateTime;
begin
  if ANow < AThen then
    Result := AThen - ANow
  else
    Result := ANow - AThen;
end;

Complicado, complicado, pues no es...