12 month sum based on current date

Hello, I have a file in which i am trying to keep a current 12 month sum of forecast values. Essentially if it is April of 2018 i want it to forecast the sum for the following 12 months as well and when a new month occurs it automatically switch to the current month and sum the new out month forecast for the next 12 as well. In the attached Columns "C" and "I" would be the columns to be summed.