Oct 15, 2016 · Let me illustrate this with a simple example. Consider an item that has more or less level sales with a lot of variability and an item that has seasonal sales with no randomness whatsoever. The first is difficult to forecast, while the second is as easy as it gets (just copy the previous season as your forecast!). For example, if the credit period is 2.5 months and the current month is April, then March and April will be “whole months” where no payment has been received, with half of February’s monies still outstanding too. The reason for the IF statement here is to prevent calculations considering periods before the beginning of the forecast period.