Assigning spend to multiple different events

(Apologies for the butchered title - couldn't think of a concise way of explaining it).

So - I have spend stored on a daily basis in each marketing channel report. Let's say this is marketing each time for a monthly event - but there's overlap between the "lead-time" for those events.

So, for example if the lead time is between 2 and 4 months of the event - then for an event in December, I'd like to factor in spend made between September and October. For January, it'd be for spend between October and November. How would I put this in a calculation?

Say I had a table with a list of events, each on the first of the month. How would I go about creating a *relative* formula that would say sum up all spend from sources A, B and C but only considering dates that were say between 120 and 60 days removed from the day of the event? Trying to come up with a solution but don't really know where to start. My main issue at the moment is that because I need a days worth of spend to be applied to multiple events, I can't work out how to do that.

Any help greatly appreciated.

Thanks,

Bob