One sledgehammer solution

=INT(LEFT(A1,FIND(" ",A1)-1)-LEFT(B1,FIND(" ",B1)-1)+(MID(A1,FIND(" - ",A1)+3,2)-MID(B1,FIND(" - ",B1)+3,2))/12)&" Y - "&INT(MOD(LEFT(A1,FIND(" ",A1)-1)-LEFT(B1,FIND(" ",B1)-1)+(MID(A1,FIND(" - ",A1)+3,2)-MID(B1,FIND(" - ",B1)+3,2))/12,1)*12)&" M"