input for timestamp

I am creating a report to capture how many SR are open at a certain instance in time. I can put in a filter and the report works perfectly but I have to hardcode the timestamp into the filter, see below;

(("- Service Request Custom Attributes".DATE_7 IS NULL AND "Service Request"."Closed Date and Time" IS NULL) OR ("- Service Request Custom Attributes".DATE_7 IS NULL AND "Service Request"."Closed Date and Time" >= timestamp '2013-09-06 17:00:00') OR ("- Service Request Custom Attributes".DATE_7 >= timestamp '2013-09-06 17:00:00')) AND "Service Request"."Opened Date" <= timestamp '2013-09-06 17:00:00'

My question is can anyone tell me how I can create a prompt to populate the timestamp for this filter. I just need one prompt because it will be the same timestamp for each instance.