Date diff nresults

How to filter a query by parameter dates , the total for Novemember 2018 minus December 2018 total , because the users will not be able to see the actual quary I have a ttached a parameter form binded to a query which has the details.

The form has two date parameter ( [start date] to [End date]) where they will fill in , for example the difference for December 2018 is calculated the following:

<= Start AND End date

The problem here isntead of start from 30/11/2018 it is start from 1/12/2018, so how do I make sure that the picked results start from 30/11/2018 isntead of 01/12/2018

SalesMonth      salary      Cumm      Commission
Jan-18      10500      10500      2625
Feb-18      10500      21000      5250
Mar-18      10500      31500      7875
Apr-18      10500      42000      10500
May-18      10500      52500      13125
Jun-18      10500      63000      15750
Jul-18      10500      73500      18375
Aug-18      10500      84000      21000
Sep-18      10500      94500      23625
Oct-18      10500      105000      26250
Nov-18      10500      115500      28875
Dec-18      10500      126000      31500

Regards

Chris