Question

I am having a weird problem with PHP’s DateTime diff method. I’ve been using DateTime methods for some while now and thought I’ve got the hang of it, but this problem amazes me.

I am using the diff method to determine the number of months/days between two dates.
Today, I used the following code:

$start = new DateTime('2012-03-01');
$end = new DateTime('2012-03-31');

$period = $start->diff($end);

var_dump($period);

Which produced the following output:

object(DateInterval)[3]
  public 'y' => int 0
  public 'm' => int 1 // Shouldn't this be 0?
  public 'd' => int 1 // And where does this come from? Would expect 30.
  public 'h' => int 0
  public 'i' => int 0
  public 's' => int 0
  public 'invert' => int 0
  public 'days' => int 30

As my comment above says, where the hell does the 1m1d come from (I would expect 0m30d). I can’t for the life of me figure out where it gets this value from. I also tried putting the midnight timestamp 00:00:00 behind both dates to make sure it wasn’t a time thing, but that made no difference to the output.

Can anybody explain this outcome to me?
This problem occurs on my Debian development server, running PHP 5.3.3 with Xdebug 2.1.1 and Suhosin 0.9.32.1 installation.

Update

Reproduced this problem on my production server, which is on CentOS 6.0 with “raw” PHP 5.3.8 without any extensions/patches.

Update 2

I managed to find a “workaround” (only working at this moment in time). If I change the absolute dates to:

$start = new DateTime('first day of next month');
$end = new DateTime('last day of next month');

It does give 0m30d as output. Still, I’m stumped by the above, so if anyone has a clue about that, please let me know!

Answer 1

Congratulations, you have just discovered DST! If you check with the DST switchover for your timezone, you will see that $start is actually getting Feb-29, and $end getting Mar-30. (2-3) = 1 (therefore, m=1), and (29-30) = 1 (therefore, d=1).