Create a ""sumif"" type function in a join table for many-to-many relationship

ok so you need to make sure that on the join table you have lookup fields from both parent tables, especially billable date, revenue, forecast date, and 1st day of forecast month.

when you create your summary field on the forecasts table, summarize the revenue field with filtering something like:

where billable date is on or after 1st day of forecast month
where billable date is on of before forecast date

try that and let me know.