If I understand correctly you want to display all ticket that have not be ack for more that 15 mn.
your timespans will return now -15mn in unixtimestamp and compare to it to open_date in unixtimestamp
then let's assume now:
now is 2016/06/09 08:15:00 converted unixtime = 1465460100
open_date is 2016/06/09 07:45 converted unixtime = 1465458300
15 duration in converted unixtime= 900
then your PAST15MINS (at the time of the evaluation) is 1465460100 - 900 = 1465459200
as you can see the number returned for the open_date is inferior to the past15MIN
so your comparaison if you want to return tixket that have past those 15 mn must be:
(group.[group]group_list.member IN (@cnt.id)) AND active = 1 AND type = \'R\' AND status=\'OP\' AND zfmdpc = 400001 AND open_date < StartAtTime(\'PAST15MINS\')
Hope this help
/J