=iif(Fields!Year.Value=2012 and Fields!month.Value=2,29,iif(Fields!Year.Value=2013 and Fields!month.Value=2,28,iif(Fields!month.Value<8 and Fields!month.Value mod 2=1,31,iif(Fields!month.Value<8 and Fields!month.Value mod 2=0,30,iif(Fields!month.Value>7 and Fields!month.Value mod 2=0,31,30)))))