Trying to get the maximum date in column M if criteria met:

=SUMPRODUCT('WaitTimes'!Q:Q=Sheet1!A2,'WaitTimes'! H:H="1)",'WaitTimes!G:G="B",??? I need to add the maximum date in column M)???

I'm trying to figure out how to get the maximum date in column M if all of
the criteria is met -- can you help me?

Thanks in advance.