project_usage_get_weeks_since() computing wrong indexes

The project_usage_process_weekly() function called by the cron job expects that the array of timestamps returned by project_usage_get_weeks_since() will begin with an index of 0. But instead it is returning an array with an initial index of 0 due to a misunderstanding on my part. I thought that $foo[$i] = $i++ would give array(0 => 0); rather than array(1 => 0);. As a result the first week in the week's worth of stats wasn't being computed correctly.