order date is taking into consideration

I setup the VAT starting from 01/01/18.  But in Sales order and Purchase order it is taking into consideration if order date >=01/01/18.  For eg if we put order date 25/12/17 and request date 01/01/18, system will not consider the VAT.  means system considering order date instead of requested date. 

Is there any option to give preference to requested date instead of order date.

Please help.