Date function for budgeting

I want to make a function that gives a rolling daily average of money from a monthly total.

So say I have $5000 allotted per month (I wish!), and the "month" I'm considering is between 5/24 and 6/24. I have another function setup that calculates how much money I have left from that monthly total by tracking expenses that I enter in a certain column. What I want to do is create a formula that will use the number of days left in the "month" that I'm considering (or any length of time really) and divide that into the remaining amount of money per "month". What I can't figure out is how to do this with excel keeping track of how many days are left in the "month", rather than me physically entering the new amount every day.

Sorry if this is unclear! Thanks for the help.

Oops, wrong section, fixing that