with dates in COL A (starting in A1)
readings in B (starting in B1)
in C1 put
=B1-(INDEX($A$1:$B$11,MATCH(A1-365,$A:$A,1),2))
and drag down
or if you dont want to see #N/A
=IF(ISNA(B1-(INDEX($A$1:$B$11,MATCH(A1-365,$A:$A,1),2))),"",(B1-(INDEX($A$1:$B$11,MATCH(A1-365,$A:$A,1),2))))