[fpc-devel] Magic numbers used in TryEncodeDate

[Graeme Geldenhuys]

What is the meaning of these magic numbers used in the TryEncodeDate method?
I would like to document them for FPC once I know the answer.

146097
1461
153
693900

I saw the constant DateDelta, but that is never used in TryEncodeDate
(whereas in Delphi it is).

   DateDelta = 693594;        // Days between 1/1/0001 and 12/31/1899

All of this came about while trying to understand why I get a
different results when I do the following under Delphi and FPC:

  dt := EncodeDate(1652, 6, 15) + EncodeTime(12, 34, 56, 12);
and then format dt to a string as "yyyy-mm-dd hh:mm:ss"

Result under Delphi 7:
 expected: <1652-06-15 12:34:56> but was: <1652-06-16 11:25:03>

Result under FPC:
 expected: <1652-06-15 12:34:56> and got it.


----------------------------------------------------------------
Function TryEncodeDate(Year,Month,Day : Word; Var Date : TDateTime) : Boolean;
var
  c, ya: cardinal;
begin
  Result:=(Year>0) and (Year<10000) and
          (Month in [1..12]) and
          (Day>0) and (Day<=MonthDays[IsleapYear(Year),Month]);
 If Result then
   begin
     if month > 2 then
      Dec(Month,3)
     else
      begin
        Inc(Month,9);
        Dec(Year);
      end;
     c:= Year DIV 100;
     ya:= Year - 100*c;
     Date := (146097*c) SHR 2 + (1461*ya) SHR 2 +
(153*cardinal(Month)+2) DIV 5 + cardinal(Day) - 693900;
   end
end;
----------------------------------------------------------------

Regards,
  - Graeme -

--
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