defer revenue across month-end

hi everyone...hope this will be simple for one of you excel geniuses...!

I have a sales item that is billed as a lump sum. (lets say it is $10,000)
included in this lump sum are three components as follows: a "build" ($6,000), a "21 day rental" ($2,000) and a "decommission" ($2,000)

When i charge my client for the lump sum i may need to defer some of the revenue for the rental & decommission components depending on when the build date is
e.g if i build on 5-March then there is no deferment as the 21 days rental all falls within the same month as the build.
e.g. if i build on 22-May then i will have to defer 11 days of the rental (last day of the month - day 22 = 11 days) and all of the decommission into my June revenue.

hope that makes sense...? Is there a simple calculation i can use to apportion the rental days across two months if i need to.? It's just the rental splitting based on dates i am struggling with due to the different days in each month
I've not done much excel with date functions before so hoping i can learn from someone clever on here...
any questions please shout back

cheers legends.!

spreadsheet example attached

updated with a better example hopefully

thanks Olly, yes i found EOMONTH function and played around with it until it worked.