Create a new SQL Agent Job, and add a Transact SQL step:
update tab_name
set due = DATEDIFF(DAY, INSERTDT, UPDATEDDT) - DATEDIFF(DAY, INSERTDT, GETDATE())
Obviously, unlike the trigger, you can't update those that have just been updated. So this will update all 'due' fields based on the time it runs.
I would consider create a stored proc and getting the job to run that instead. It easier manager, and less likely to get missed in the future.