how 2 calculate Interest on late payment of VAT

Hi friends...my client falls under quarterly dealer catagory and for the quarter ending Dec'10...so i think before 22nd Jan'11 he should pay tax for the quarter ended Dec'10....his laibilty is say of Rs.1200/- but  he has not paid the same ...so can any1 let me know how 2 calculate interest ...i know rate of interst is 18 % p.a but shall i take it on monthly basis or on daily basis of default ?