Why my User Defined Field does not refresh when other field changes

Hi Experts one make a UDF and a Query which compute fig of days betwixt 2 day utilizing DATEDIFF and one privation to amusement it to UDF Field, one setup it utilizing FMS erstwhile AR Invoice owed day tract is alteration it volition automatically compute to the UDF tract the full Difference of my Posting Date and Due Date, but seemingly it does not refresh astatine all. present is simply a illustration of my code.