The attached PlanGuru model shows one way you can introduce a month by month variability into your Average Days to Collect or Average Days to Pay metrics. Using the methods demonstrated here will allow you to further refine your A/R and A/P calculations.
You will notice that 2 A/R categories are on the balance sheet. One is created using the standard average days to collect method. The other is created using the non-financial data tab. Both are set up for the purposes of compare and contrasting the two approaches.
NON COMMENTED CATEGORIES ARE NOT PERTINENT TO THIS DEMONSTRATION. Start by reviewing the commented categories (red box in the upper right hand corner of the category description) in the Non-Financial Data tab. Each successive commented category will show the logic I used to introduce variability into the Average Days to Collect.
The "% Sales Collected Current Period" is where you can adjust your average days to collect per month. Notice that I have changed this amount in November, this will change the projected A/R balance for Nov & Dec.
I've demonstrated a general technique using A/R, however the same logic could also be used to calculate A/P with a variable average days to pay.