Description:
------------
PHP has a function called mktime()
(see http://www.php.net/manual/en/function.mktime.php)
this uses internally the Unix/Linux mktime() function.

We detected the problem within the phpGroupWare
(actual cvs checkout from branch 0.9.16)

www.phpgroupware.org 
cvs repositories:
https://savannah.gnu.org/projects/phpgwapi
and
https://savannah.gnu.org/projects/phpgroupware/
 

The bug itself will occur in the following file: 
http://savannah.gnu.org/cgi-bin/viewcvs/phpgwapi/phpgwapi/inc/class.datetime.inc.php?rev=1.5.2.12.2.8&content-type=text/vnd.viewcvs-markup

in the method 

function get_weekday_start($year,$month,$day)

when php mktime() will be called, the result (in seconds) will be off by 2 days. This happens especially when zero or negative day values will be used.
These values are allowed by php mktime() as well as by the underlying unix mktime()
(see Itanium man mktime).

We found that the problem only occurs in the above file,
using the same values in a small test script will give backe the correct results!

We assume that it might be a memory/stack problem within php itself, that we are not able to track down.


Reproduce code:
---------------
For URLs see description.

switch($weekday)
{
  case 0:
    $sday = mktime(2,0,0,$month,$day - 6,$year);
    break;
  case 1:
    $sday = mktime(2,0,0,$month,$day,$year);
    break;
  default:
    $sday = mktime(2,0,0,$month,$day - ($weekday - 1),$year);
    echo ':' . $day . ':' . $weekday . "<br />\n";
    echo ':' . $year . '/' . $month . '/' . ($day - ($weekday - 1)) . ' : ' . $sday . ":<br /><hr />\n";
}


Expected result:
----------------
The correct value in seconds (differs a bit by tomezone settings), but not values that are 2 days or more off.

orrect Output values here 32bit linux machine:
 
:1:4
:2004/4/-2 : 1080518400:
:1:2
:2004/6/0 : 1085961600:
:1:6
:2004/05/-4 : 1082937600:
:1:6
:2004/05/-4 : 1082937600:


wrong output values 64 bit itanium2:
 
:1:4
:2004/4/-2 : 1080694800:
:1:2
:2004/6/0 : 1085965200:
:1:6
:2004/05/-4 : 1083286800:
:1:6
:2004/05/-4 : 1083286800: