format_date vs date_format_date drupal 7
Drupal 6/7 core:
// $datetime example "2009-12-09 11:57:16"
$timestamp = strtotime($datetime);
$my_date = format_date($timestamp, $type = 'medium', $format = '', $timezone = NULL, $langcode = NULL);
Drupal 6 with date module:
$date = date_make_date($timestamp, $timezone = NULL, $type = DATE_DATETIME, $granularity = array('year', 'month', 'day', 'hour', 'minute'));
$my_date = date_format_date($date, $type = 'medium', $format = '', $langcode = NULL);
Drupal 7 with date module:
// $datetime example "2009-12-09 11:57:16"
// Construct own format: DateTime::createFromFormat
$date = new DateObject($datetime);
$my_date = date_format_date($date, 'custom', variable_get('mycustomtype', 'maand'), $langcode = NULL);
Notice how you can't use predefined configurations directly in $type as is possible in drupal core's format_date.
Related issue: http://drupal.org/node/1343464