for nerds or even a bug: negative date works "sometimes"

[Nicole]

Here is a thing, I want to share with you.
To find a workaround is easy, but this is too strange to just work around.

All starts with a line in an iCal or *.ics

Code: Text  [Select][+][-]

1. DTSTART:16010311T020000

Who is not familiar with it: This means the 11th of March in the year 1601 at time 2 o'clock.
So a date-typo, ok
We reduce it to
16010311
which is in the year 1601.

This I wanted to skip by a validity check, but surprisingly it passed.
I wrote:
valid:=TryStrToDate(s, myDate);
Valid was true and the date -109105.

A reverse check
ShowMessage(DateToStr(myDate));
gave me the correct date in the year 1601.
WOW!

The strange thing starts, when I add the value to a Listbox

I added it by
ListBox1.Items.Add(DateToStr(newsA.datum)

The value newsA.datum is negative, and contains the 17the century-date.
However the displayed Listbox contains
30.12.1899
instead, what shall be zero.

Awaiting any comments.

[marcov]

Afaik some negative date issues have been fixed in both 3.2.3 and trunk

[wp]

I don't understand this post. I have completely different experience...

valid:=TryStrToDate(s, myDate);
Valid was true and the date -109105.

Please show the complete code how you did this. In my opinion it should not work: first, because you do not specify a formatSettings parameter, and this means that your DateSeparator (probably a '.', '-', or '/') is expected to be present in the string. Since it is not present, the conversion should fail. Secondly, StrToDate and TryStrToDate absolutely need a dateseparator to find the end of the year and month digits. I debugged it and the accumulation of the y's did not stop after 4 digits.

It works, however, with the function ScanDateTime which only seems to count the number of Ys, Ms and Ds to get the year, month and day digits.

Code: Pascal  [Select][+][-]

1. program project1;
2.  
3. uses
4.   SysUtils, DateUtils;
5. var
6.   s: String;
7.   dt: TDateTime;
8.   fs: TFormatSettings;
9. begin
10.   s := '16010311';
11.  
12.   fs := DefaultFormatSettings;
13.   fs.ShortDateformat := 'yyyymmdd';
14.   //fs.DateSeparator := #0;  --- there's no way to set the date separator to '' since it is a char rather than a string...
15.  
16.   WriteLn('TryStrToDate');
17.   if TryStrToDate(s, dt, fs) then
18.   begin
19.     WriteLn(s, ' ---> ', dt:0:0, ' ---> ', DateToStr(dt) )
20.   end else
21.     WriteLn('invalid date');
22.  
23.   WriteLn;
24.   WriteLn('ScanDateTime');
25.   try
26.     dt := ScanDateTime('yyyymmdd', s, fs);
27.     WriteLn(s, ' ---> ', dt:0:0, ' ---> ', DateToStr(dt) )
28.   except
29.     on E:Exception do WriteLn('invalid date');
30.   end;
31.  
32.   ReadLn;
33. end.

The strange thing starts, when I add the value to a Listbox

I added it by
ListBox1.Items.Add(DateToStr(newsA[ i].datum)

The value newsA[ i].datum is negative, and contains the 17the century-date.
However the displayed Listbox contains
30.12.1899
instead, what shall be zero.

In my own test DateToStr does accept a negative date without issues. Please post a short program so that we can see what you are doing.

[Nicole]

just posted with all my work inprogress inside.

Code: Pascal  [Select][+][-]

1.  for i:=0 to SL.Count - 1 do begin   // 2.3.2024 J ist bei der Zeit - 1 und die ChecklisBox ist uU die falsche Komponente
2.      Zeile:=SL[i];
3.      if ContainsText(Zeile,'DTSTART') then begin   // DTSTART:20240112T120000  => das ist 12 Uhr, was wäre EST nicht CST?
4.         teil:=trim(Zeile);
5.         k:=Pos(':', teil);  // 1.6.2017 daher suche ich nach dem Doppelpunkt
6.         teil:=copy(teil, k+1, Length(teil));
7.         teil:=trim(teil);
8.         s:=leftStr(teil,8); // 20240112T120000 =>   20240112
9.         s:=copy(s,5,2)  + '.' + copy(s,7,2) +'.' + copy(s,1,4);  // Datum wird verwandelt in  12.1.2024
10.         gueltig:=TryStrToDate(s,datum_);
11.         if gueltig  and (datum_ > 0) then begin // nur wenn das Datum gültig ist, wird etwas eingetragen
12.            j:=j + 1;   // für eine neue Zeilennummer im Array newsA
13.            newsA[j].Datum:=datum_;  // damit habe ich den Eventtext extrahiert
14.             // wie kann das negatig und gültig sein?!
15.         //   ShowMessage(DateToStr(datum_)); Das "problem" ist, dass das korrekt verarbeitet wird.

Code: Pascal  [Select][+][-]

1. type TNews = record
2.   id_news: integer;
3.   datum: TDate;
4.   Zeit: TTime;
5.   Code: integer;
6.   Bezeichnung: string; // max 80 Zeichen in der Tabelle
7.   end;
8.  
9. TNews_array = Array of TNews;  

To my mind the negative date is an advantage, because it enables to display data before 1800.

PS: If you see any connection with your kind e-mail, that ical is nothing but text, - you are perfectly all right with this idea.

[Nicole]

and the iCal, I am afraid you know it.
However it is not about the iCal, it is about writing data before 1800.

[wp]

In the particular case of the ical file, you could also use TryISOStrToDate or TryISOStrToDateTime (rather than TryStrToDate), they are able to work without a date separator.

And if you plan to use negative date in TvPlanIt: I just found out that there is a limitation in the 17th century because then the formula used to calculate Easter is not valid any more.