[fpc-pascal] date formating dificulty

[Michael Van Canneyt]

On Tue, 18 Nov 2014, Philippe wrote:

> 
> Em 18.11.2014 09:49, Philippe escreveu:
>
>       I wrote a program:
>
>       - windows 8 32 bits
>
>       - lazarus 1.2.4  ... fpc 2.6.4
>
>       then I copied it to another computer
>
>       - windows 7 64 bits
>
>       - lazarus 1.0.14 .... fpc 2.6.2
>
>       and got into trouble with dates format! programs behave diferent way, and what was working on the fisrt computer does not work
>       properly in the second!
>
>       I guess I need some tutorial (I looked for but did not find anything :( !)
>
>       why formatDateTime( 'DD/MM/YYYY', now) returns 18-11-2014 and not 18/11/2014 ?
>
>        
>
>       strtodate( formatDateTime( 'DD/MM/YYYY', now)) works ok on the w 8 32 bits, but arise an exception on the w 7 64 bits.
>
>        
>
>       I am confused with the following syntax
>
>       function FormatDateTime(
>
>         const FormatStr: string;
>
>         DateTime: TDateTime;
>
>         const FormatSettings: TFormatSettings
>
>       ):string;
>
>       how does it work between two format especifications: FormatStr and FormatSettings ?
>
>       appreciate any example of good programming pratise of data formating to avoid bad surprise moving program from an OS to another !
>
>       Philippe
>
>        
>
>       on http://www.freepascal.org/docs-html/rtl/sysutils/strtodate.html
>
>       I found
> 
> Program Example19;
> 
> { This program demonstrates the StrToDate function }
> 
> Uses sysutils;
> 
> Procedure TestStr (S : String);
> 
> begin
>   Writeln (S,' : ',DateToStr(StrToDate(S)));
> end;
> 
> Begin
>
>   Writeln ('ShortDateFormat ',ShortDateFormat);
>   TestStr(DateTimeToStr(Date));
>   TestStr('05'+DateSeparator+'05'+DateSeparator+'1999');
>   TestStr('5'+DateSeparator+'5');
>   TestStr('5');
> End.
> 
>
>       the compiler warns that ShortDateFormat is deprecated ... which is not informed in the doc ...

It should be, I will check.

Anyway, as Graeme hinted: ShortDateFormat contains a format string, and / is a placeholder for the date 
separator character on the computer on which the program is running.

If you want to force a character, you can enclose it in quotes:
ShortDateFormat:='dd"/"mm"/"yyyy'
will always use / as the date separator, no matter what the internatonalization settings on the PC.

But it is bad practice to do so.

Secondly, it is a common mistake to think that StrToDate will always correctly perform the opposite of DateToStr.
It does not, but this is documented.

Michael.