automatic insert date without change in future with VB code

dear
i have problem with insert automatic insert date
as in attached file i want to add date in I6 when i write value more than 0 in I2
and repeat this from J2 to AF2

and also want to repeat this with another in I8 (write any value more than 0 and date automatic insert to I12
i already made it but it give me error as you can test .

and the last request is
in cell E2 have number (12 or 18 or 24 manual entry), my question is how i connect the number of cells from I4 to AF4 (meaning if in cell E2 write 12 its only allow to wrote function from I4 to T4 (meaning 12 cells only but if 18 its allow to Z4 and soon without change in function of this cells.
hope you can understand me

Dear AlphaFrog
thanks for your help and your effort
would you please explain how can add extra cell in this sheet because i try to add more than ROW with the same function but dosn't work (meaning repeat this data in excel )
if you can explain to me how can understand this formual you write i think i can add extra row or delete any row ?
regards

i mean as you see in attached file
you make function for data in row 1:6
and data in row 7:12
i need to add extra row and apply this function on it as in attached file

hope you understand me now

ok thanks Alphafrog
its work now