PHP strtotime is wrong (some times)?

Question

I have this issue with strtotime in my PHP code that it is wrong sometimes (only for some timezones) and it is correct for others!!

I cannot get my head around it.

I have set the <?php date_default_timezone_set('GMT'); ?> at the top of my page as well but that doesn't help!

basically what it does is that it will add or subtract the offset/3600 value to the set time in $time1 = strtotime('00:00'); depending on the if and else if conditions.

the offset/3660 value is the time difference between two timezones!

the code bellow works for some locations and it doesn't for others! basically it will add an extra 1-2 hours or takes off/subtract an extra 1-2 hours some times (not all the time).

i.e. the time difference between Abidjan and london is -1. the time (value) that should be shown is 23:00 as 00:00 - 01:00 = 23:00. but the value is shown is 00:00.

However as i mentioned it works for some timezones. i.e. for the time difference between New York and London which is -5 the code works and it shows 19:00 as 00:00 - 05:00 = 19:00

could someone please shed a light on this?

here is the code in question:

<?php
$time1 = strtotime('00:00');

if (0 > $offset)
{
   // For negative offset (hours behind)
  $hour_dif = date('H:i', strtotime($time1 -$offset/3600));
  $time1 = "{$hour_dif}";
}
elseif (0 < $offset)
{
   // For positive offset (hours ahead)
     $hour_dif = date('H:i', strtotime($time1 +$offset/3600));
     $time1 = "{$hour_dif}";

}
else
{
   // For offsets in the same timezone.
   $time1 = "in the same timezone";
}

echo "{$time1}";
?>

Answer 1

Well, since strtotime() already returns a timestamp and date() expects one you could just do

$hour_dif = date('H:i', ($time1 - ($offset*3600)));

or

$hour_dif = date('H:i', ($time1 + ($offset*3600)));

respectively, to remove or add the correct amount of seconds from/to the timestamp.

I am also assuming that $offset is the offset in hours, so you would have to multiply by 3600 to get the amount of seconds, not divide.

---

Alright, after testing your code and thinking through it some more, it became obvious.

With a negative offset like -1 you're going to calculate $time1 - (-1) * 3600 and we all know that a double negation is positive...

So in fact, your code can be condensed to:

$time1 = strtotime('00:00');

if ($offset == 0)
     $time1 = "in the same timezone";
else
{
   // For positive offset (hours ahead)
     $hour_dif = date('H:i', ($time1 + ($offset*3600)));
     $time1 = "{$hour_dif}";
}

echo "{$time1}\n";

and should work as expected:

cobra@box ~ $ for i in {-24..24}; do php test.php $i; done;
00:00
01:00
02:00
03:00
04:00
05:00
06:00
07:00
08:00
09:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
18:00
19:00
20:00
21:00
22:00
23:00
in the same timezone
01:00
02:00
03:00
04:00
05:00
06:00
07:00
08:00
09:00
10:00
11:00
12:00
13:00
14:00
15:00
16:00
17:00
18:00
19:00
20:00
21:00
22:00
23:00
00:00