timestamp calculations

BurfTone

Hi,

To calculate the number of days between timestamps I've divided the difference by 86400 (secs in a day). I've seen this recomended in other posts. The timestamps have all been created with mktime using 0,0,1 for Hour, min, secs.

Certain months give non integers as answers and then it corrects itself once it is past that month. The code below will reproduce this behavior. $calcval is the correct number of days between stamps (it takes the ceil of the calculation) $calcval2 shows the weird values.

Any idea what causes this?

Thanks!

$loopstamp=mktime(0,0,1,DATE("n"),DATE("d"),DATE("Y"));
$first_loop=$loopstamp;
for ($i=0; $i<700 ; $i++)
{
$calcval=ceil(($loopstamp-$first_loop)/86400);
$calcval2=($loopstamp-$first_loop)/86400;
echo DATE("Y-m-d H:i:s",$loopstamp) . ' and ' . $calcval . ' and ' . $calcval2 . '<br>';

$loopstamp=mktime(0,0,1,DATE("n"),DATE("d")+$i,DATE("Y"));

}

devinemke

your problem is that you are only incrementing the DAY in your loop, you are not incrementing the month.

consider this example:

$start = strtotime('today 00:00');
$end = strtotime('today 00:00 + 1 year');
$date = $start;

while ($date <= $end) 
{
	$diff = ($date - $start) / 86400; 
	echo date('Y-m-d', $date) . ' is ' . $diff . ' days from today<br>';
	$date += 86400;
}

BurfTone

Hi,

mktime will automatically increment the month when the days added go into the next month. When adding 86400 directly to each day, everything works fine. It is when using mktime that the problem occurs. Here is a one day example that shows the same problem.

$start_stamp=mktime(0,0,1,1,11,2005);
$end_stamp=mktime(0,0,1,4,4,2005);
$days_between=($end_stamp-$start_stamp)/86400;
echo $days_between . ' this is the weird value<br>';

Thanks.

Mucello

The project I'm working on now (and have been for about a year) utilizes timestamps heavily. So I've been down this road many times.

The one thing I want to offer to this discussion is a small warning flag. If you are running this script on a 32 bit machine, you'll probably run into great difficulties and oddities with dates prior to 1970. Not many people are working on projects with data that far back, but if you are and if that happens, don't be surprised.

devinemke

Originally posted by BurfTone
mktime will automatically increment the month when the days added go into the next month. When adding 86400 directly to each day, everything works fine. It is when using mktime that the problem occurs. Here is a one day example that shows the same problem.

$start_stamp=mktime(0,0,1,1,11,2005);
$end_stamp=mktime(0,0,1,4,4,2005);
$days_between=($end_stamp-$start_stamp)/86400;
echo $days_between . ' this is the weird value<br>';

your code produces "weird values" because you are not considering Daylight Savings Time (which starts at midnight 04-03-05)

BurfTone

your code produces "weird values" because you are not considering Daylight Savings Time (which starts at midnight 04-03-05)

AHA! That is why it only occurs over certain months and the change reverts back after a while!

Thanks!