使用DateTime diff的DateInterval错误

Question

I'm comparing two DateTime :

//Get the current DateTime
$date_now = new DateTime;
var_dump($date_now); // 2014-03-10 19:04:29

// I need to subtract 12 hours (I'm using $date_past to fetch $db_date in a request)
$date_past = $date_now->sub(new DateInterval("PT12H"));

// Get the DateTime from $db_date just fetched in database
$alert_date = new DateTime($db_date);
var_dump($alert_date); // 2014-03-10 17:04:00 in my test

// Get the difference
$diff = $alert_date->diff($date_now);
var_dump($diff->format("%H:%I:%S")); //09:59:31

So I get 09:59:31 and also $diff->invert == 1 which means that it's a negative value.

I saw this issue : php datetime->diff is calcualting wrong amount of hours (3 too much) and I think it's similar problem but I can't find a solution to keep using ->sub(). Any ideas ?

If you think it could be a TimeZone problem, I checked all my DateTime and they all have a TimeZone set to "Europe/Berlin", so I don't think it come from here.

Thank you !

Answer 1

The problem is that when you assign an object to a variable, it is assigned by reference. So when you do:

$date_past = $date_now->sub(new DateInterval("PT12H"));

The variables $date_past and $date_now both point to exactly the same reference and when you modify one, you modify both. You will notice this if you simply echo out the variables:

$date_now = new DateTime;
$date_past = $date_now->sub(new DateInterval("PT12H"));

echo $date_now->format('c'); // 2014-03-10T00:38:56-06:00
echo $date_past->format('c'); // 2014-03-10T00:38:56-06:00

Notice how they both print exactly the same thing. Normally what you want to do is use clone:

$date_past = clone $date_now;
$date_past->sub(new DateInterval("PT12H"));

echo $date_now->format('c'); // 2014-03-10T12:41:20-06:00
echo $date_past->format('c'); // 2014-03-10T00:41:20-06:00