GST & Bill of Entry

Neha Kankani (Service) (67 Points)

19 August 2017  
My import purchase invoice is dated 15/7/17
Bill of Entry is dated 29/7/17
Custom duty is paid on 4/8/17
At what date should I pass entry in my books?
On Bill of Entry, IGST @ 12% is charged on Assessable Value say rs. 100 i.e. GST Rs. 12 but my Actual value is say Rs. 80 in books so how should I adjust my GST?