Carryover End of Month Balance at Beginning of Each Month

Hello! I use this site all the time but this is my first post. I inherited a very basic excel workbook to keep track of client's ad spend on a month-to-month basis. At the end of each month, I've been manually taking the leftover budget and adding it as a negative value in the next month to carry the balance forward. I know there has to be a formula that would automatically carry over this balance each month but can't figure it out. I have all of my clients listed by row with their monthly budget total in a column (J). I input any spending in worksheet "Register"." I've attached the workbook I am dealing with. Since it is the 1st, all of the values have reset. To see the values from last month, I would put in 11/30/2016 into cell O18. Any help or insight would be greatly appreciated. Thanks!