Jessie, I think it is possible with a custom time stamp...you could copy the PAST_MONTH and modify it and then use
the new one instead of PAST_MONTH. By the way, the query should be modified to as
group.[group]group_list.member IN (@cnt.id) AND completion_date > StartAtTime(\'PAST_MONTH\') AND completion_date < EndAtTime(\'PAST_MONTH\') AND status = \'COMP\'
The one I suggested before returns all complete tasks before this month, not only last month's
Thanks _Chi