There are a couple ways to accomplish this. One would be a scripting extension that you have to manually run, but which can spit out your updated value every day.
Another way would be to create a View that filters to only show records from the last 30 days:
And then just look at the summary bar at the bottom every day to get your updated value (records that are 31 days old and older will automatically drop off the View at Midnight in the selected Timezone).
If you don’t want to have to go reference the View for your value, you could combine that filtered view with a Summary extension that Sums the value from a field for all records in that view:
Those are probably your best options, although there are others.