month=`date +%m`
day=`date +%M`
year=`date +%Y`

count=13

while [ $count -gt 0 ]
do
    count=`expr $count - 1`
    month=`expr $month - 1`
    if [ $month -lt 1 ]
    then
        month=12
        year=`expr $year -1`
    fi
done

compare_date="${year}${month}${day}"

while <***read the file***>
do
    if [ $DATA_VENCIMENTO -lt $compare_date ]
    then
        <***do stuff***>
    fi
done