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Assuming the date you want to test is in A1, write
=if(median(A1,"12/1/11","3/31/12")=A1,"Prior Period",if(median(A1,"4/1/12","5/31/12")=A1,"Current Period","Outside Range"))
Vist http://chandoo.org/wp/2010/06/24/between-formula-excel/ for more info on...