Date difference

The syntax to create a new date object is:

$date = new DateObject($value, date_default_timezone(), DATE_FORMAT_ISO);

The following code sample shows how to calculate the difference in days between today and the value in a node's date field that was saved in ISO format:

$date_value = field_get_items('node', $node, 'field_date')[0]['value'];
$fieldDate = new DateObject($date_value, date_default_timezone(), DATE_FORMAT_ISO);
$nowDate = date_now();
$diff = $fieldDate->difference($nowDate, 'days');

See DateObject::difference() for more information on that method.

If your date is in a default Date field, you would need to use DATE_FORMAT_DATETIME instead of DATE_FORMAT_ISO.