I'm trying to calculate the value between two fields in my custom node.tpl file. I am trying to convert the code show on http://drupal.org/node/1364744, and I wrote the following one.

$birthday = format_date(strtotime($content['field_date_de_naissance']['#items'][0]['value']), 'custom', 'd/m/Y');
$today = date('m/d/Y');
$age = $birthday->difference($today,'years');
print $age;

I get this error.

Fatal error: Call to a member function difference() on string in

What is wrong in my code?


2 Answers 2


The error is calling a method (difference()) on a string, while just PHP objects and classes implements methods. format_date() is documented to return a string, not a DateObject.

Return value

A translated date string in the requested format.

The code I would use depends from what modules you have installed, and which PHP version you are using.

  • If you have the Date module installed, and you enabled the Date API module, I would use the following one.

    $birthday = new DateObject($content['field_date_de_naissance']['#items'][0]['value']);
    $age = $birthday->difference(date_now(), 'years');
  • If you don't have the Date module installed, but you have a recent version of PHP 5 (which means higher than 5.2.0), then I would use code similar to the following one.

    $birthday = new DateTime($content['field_date_de_naissance']['#items'][0]['value']);
    $age = $birthday->diff(new DateTime())->format('y');
  • If you are still using a PHP version lower than 5.2.0, or you just want code you can use without to worry about the PHP version, I would use the following code.

    $birthday = strtotime($content['field_date_de_naissance']['#items'][0]['value']);
    $age = format_interval(time() - $birthday, 1);

    I am not using format_date() since it returns a translated string, which is not necessary to strtotime() (and probably, it would confuse it).
    format_interval() consider a month made of 30 days; for this reason, the code shown above is probably giving a different value than the other methods.


  • The funny thing that your code give me 20 years for $birthday=01/12/1994, where I get 21 years when I use my code (see it in the answer). can you explain me plz?
    – learner123
    Commented Sep 7, 2015 at 17:13
  • In the comments of format_interval function documentation api.drupal.org/api/drupal/includes%21common.inc/function/… I read that it's consider 30 days as 1 month.
    – learner123
    Commented Sep 7, 2015 at 17:22
  • Yes, it consider a fixed length for a month (2592000 seconds).
    – apaderno
    Commented Sep 7, 2015 at 20:51
  • so I should use the difference() method to get the exact value.
    – learner123
    Commented Sep 7, 2015 at 22:25
  • 1
    @DanielWaters It's not possible to dispute what the OP chooses as accepted answer, but in the case of an accepted answer written by the user who asked the question, up-voting the question you think is better can make it appear before the accepted one. In any case, the score of an answer is a clear signal the users prefer an answer instead of the others; it should mean something to the future readers.
    – apaderno
    Commented Sep 16, 2015 at 5:38

I modified my code by converting the $birthday to dateObject, so it works now.

$birthday = new DateObject($content['field_date_de_naissance']['#items'][0]['value']);
$today = date_now();
$age = $birthday->difference($today,'years');
print $age;

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