Hello, Airbornm3 and thank you for posting!
You can add a rule to the field with a condition to check if it is the first day of the month (substring(field, 4, 2) = "01") and if so, just set it to a value that updates it with another function, concat(substring(field, 1, 4), "31",substring(field, 6, 10)).
One problem is that some months are not 31. So you may need to have multiple conditions based on the month, but it is not hard to add these kinds of rules and calculations.
Best of luck!