Following instructions are the product of a forum discussion taken in Drupal Israel site: http://www.drupal.org.il/node/3749 and are based on Mooffie's code with help from Yuval Hagger.
1. In Computed code paste the code that you find here:
2. In Display format paste this:
$display = $node_field_item['value'];
3. If you choose to store it in the database, varchar with 25 characters will be fine.
Note: For English names and calculation of the Jewish data look inside the code for "ELUL".