[totaltec]

Well, I would have to test it out and figure it out by trial and error myself.

1 week = 604800 seconds. I expect all of the values being processed by this script are in seconds.

This defines the current week, it simply adds another entry to the array $start_dates:
$start_dates[] = func_prepare_search_date($start_week) - $config['Appearance']['timezone_offset']; // Current week

You need to do the same. I would attempt first to simply use the equation above, and subtract 604800 seconds.

Why not:
$start_dates[] = func_prepare_search_date($start_week) - $config['Appearance']['timezone_offset'] - 604800; // Last week