Difference of Measures

Hi,

I have a table with values for 12 months(fld1...fld12) like

FLDA YEAR  FLD1 FLD2 FLD3........FLD12
A     2013  10  20 ................21
B     2012  5   ...................3

I want to have cal of monthly diff like fld2(Feb) = fld2-fld1 (feb-jan)

FLDA YEAR  FLD1 FLD2 FLD3........FLD12
A     2013       7  10 ..................

The fld1 of  Year(2013) would be fld1(10) of 2013 - fld12(3) of 2012(3) = 10-7 =3

The fld2 to fld12 is straight forward but for fld1 i have to go to previous year

I just want to see what would be the best approach for this

Thanks