You must provide a backfill date in the format YYYY-MM-DD to use backfill_date.php
example: backfill_date.php 2002-04-27 alt.binaries.games.xbox true
This will backfill your index with everything posted to a.b.g.x since April 27, 2002 that matches system regex
If you choose not to provide a groupname, all active groups will be backfilled.

If you do not want to use a date, use the backfill.php script instead.