With the pandemic, we saw a lot of libraries put a lot of near-future closures into their calendars. For libraries where useDaysMode prevents due dates on closed days, this meant a lot of checkouts with due dates that wouldn't have been allowed were the closures in place before the checkout happened. It would be helpful to have a command line script to recalculate due dates based on the current calendar and the useDaysMode syspref.
Would you also need to notify the patrons that their due dates have been extended due to updated closures perhaps?
If this theoretical script creates a clear data trail of the change somewhere, then one could easily enough use the patron_emailer cron to generate a notice for this without having to code a whole new thing for it.