Re: Date processing

[Benedikt Stefansson]

Hi Steve,

In my experience the C time library is really messy to work with. For
example I can't for the life of me understand why this program crashes
with a seg fault:

#include <time.h>

int
main(int argc,const char ** argv)
{
  struct tm *tm1;
  struct tm *tm2;
  double diff;
  char * c;

  c = (char *)strptime("1/1/1900","%m/%d/%Y",tm1);
  c = (char *)strptime("11/15/1999","%m/%d/%Y",tm2);
  diff = difftime(mktime(tm1),mktime(tm2));

  printf("Difference is %f\n",diff);

}

Besides, even if it did work the difftime function returns the number in
seconds between the two dates. One could of course get back the days as
diff/60/60/24 but I would be loath to trust that.

By a weird coincidence I spent last night trying to figure out how to do
complicated date manipulations in a Swarm program. My main concern was to
be able to work with date strings directly (11/15/1999 or 11-15-1999),
i.e. without having to manipulate the tm structs.

While this is not an Objective-C solution the quick and dirty way I came
up with was to 'borrow' a library in pure C which is used in a great Perl
module called Date::Calc. It is thoroughly documented in the man page for
that module (available from CPAN or any of it's mirrors).

The library (by Steven Beyer) is released under the GNU LGPL. I've
butchered a file from the source code for DateCalc.c and associated
header file which I called timefunctions.c (attached in tarball). I've
also implemented a new function which returns the number of days between
two date strings:

long int
DateCalc_Delta_Days_Str(const char * date1,const char * date2));

The timetest.c example program in the tarball shows how to do the
calculation. You could do your date calculations in main.m or call the C
functions from any object.

To run the example and calculate the days elapsed since January 1st 1900
and today's date save the timetest.c file and the timefunctions.c:

$ gcc -o timetest timetest.c
$ timetest 11/15/1999

For anyone using this I recommend downloading the Date::Calc Perl module,
even if you don't use Perl the man pages are quite useful. Also bear in
mind the LGPL.

Benedikt


"M. Lang / S. Railsback" wrote:

> Has anybody got an object that converts a date (Year, Julian day; or
> Year-month-date) to a standard integer format (days since 1/1/1900;
> etc.) as in Excel?
>
> And vice-versa, of course?
>
> Thanks
>
> Steve
> --
> address@hidden
> Lang, Railsback & Assoc.
> 250 California Ave., Arcata CA 95521
> 707-822-0453; Fax 822-1868
>
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timetest.tar.gz
Description: GNU Zip compressed data