Hi,

Doing your cash forecast, you are predicting, on the basis of the known and predicted transactions, what will be your cash flow in the future. You should then consider all existing open invoices (supplier and customer) and all invoices you can assume will be created or will arrive. 

Of course short term prediction is more reliable than long term one, so you can define various buckets pattern reflecting the quality of your prognosis, depending on number of days to planned date of the flow. These patterns (forecast time scale) are uysed in the cash flow forecast reporting.

Simplest is linear time scale (next 30 days, next 31-60 days etc.), but often in prognosis different scale is used - where the longer is number of days to the due date, the wider is the bucket (e.g. 1st bucket is next 10 days, 2nd - 10 to 30 days, 3rd - 30-90 days etc.) reflecting decreasing reliability of prediction. 

I hope it helps.