So youa re grouping on employee first and then "quarter" after that?
I think you could group on a formula field that would make a standardized "floating quarter" from the hire date.
I still am not sure exactly how you are calculating that so this is a stab but I hope it helps you think it through...something like...
if table.date in hiredate to dateadd("m",3,hiredate) then "Q1" else
if table.date in dateadd("m",3,hiredate) to dateadd("m",6,hiredate) then "Q2" else ...
Does this make sense?