I want to add a counter and add 70 in this date 20180101 example is when counter is greater than 31 add 70 in date 20180101 else j+1

I am setting up to add a counter in my shell script and add 70 in this date 20180101 example is when the counter is greater than 31 add 70 in date 20180101 else j+1. when we add 70 it will 20180201 again add 70 when the counter is > 31.

j=20180101 while [ $j -le 181231 ] do

            fi
j=$(( j + 1 )) #increase number by 1

 done