Hello Keith Matthews -
We are using a site workflow to do the same thing; check a list and see if any item's due date + reminder time = today.
I set it up so that the list item is running on a state machine and one of the states is "escalated" which handles all of the actions when in that state. I use the site workflow to simply check dates and then if needed, flip the item's status/state.
As mentioned above, if you are looking to check a date column contains data, you need to see if the date submitted filed is GREATER THAN 01/01/1900 because the filed is not really NULL.
Hope this helps!