how to handle GST effect after GST implementation as after 15/11/2017 new GST % will be applied on existing goods earlier was different.

Required solution / suggestion urgently on below transaction, 
  
There was a Purchase order created on 11-11-2017, with GST rate : 18 %. 

 On 15-11-2017, there was change in GST rate to 12 %. Then Material /Goods received at Factory on 20-11-2018 with Bill dated 13-11-2017 and GRN was made on same day. 

 Now at the time of Posting Invoice, i.e. on 25-11-2017, System is picking GST rate as 12 %, though Document date in mention 13-11-2017. 

 System should calculate GST rate as per Document date mention by the User, not from the Posting date.