for ($i = 0 ; $i< 365; $i++){
$date = mktime(0,0,0,1,$i,2001);
$date1 = mktime(0,0,0,1,$i-1,2001);
if (($date - $date1 ) /3600 <> 24){
echo "<br>" .date("Y m d", $date1) . " ";
echo ($date - $date1 ) /3600 ." h";}
}
It is not explicite that the mktime function take into account the change of hour some day in the year. The consequence is that (a date return by mktime )- 86400 is not always the day before.