An easier way of getting difference between dates is achieved using the DateObject provided by Drupal.
<?php
$now_date = date_now();
$last_donation_date = new DateObject($node->field_date[LANGUAGE_NONE][0]['value'], 'UTC');
// Get difference in days in positive if $now_date is greater than $last_donation_date
$diff_days = $last_donation_date->difference($now_date, 'days', FALSE);
?>
Print a Date Object in any format using date_format_date:
<?php
print date_format_date($date_object, 'custom', 'Y m d');
?>
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Source
http://drupalcontrib.org/api/drupal/contributions!date!date_api!date_api.module/function/date_now/7
http://www.drupalcontrib.org/api/drupal/contributions!date!date_api!date_api.module/function/date_format_date/7
https://drupal.org/node/1364744
Earlier PHP Date Tutorial: http://dhongibaba.jagriti.co.in/content/dates-using-php