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

Question

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?

Answer 1

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.

Reference

• DateTime::__construct()
• DateTime::diff()
• DateInterval::format()
• format_date()
• format_interval()

Answer 2

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;