You dont need FM for this.
DATA: lw_datum type sy-datum,
lw_output type char4.
lw_datum+0(4) = sy-datum+0(4).
lw_datum+4(2) = '01'.
lw_datum+6(2) = '01'.
Now, add your number to this date.
lw_datum = lw_datum + 40.
lw_output = lw_datum+4(4).
Cheers,
VS