Hi SkyTriv,
If I was building out a formula like this I would start with something like this in a formula text field, this will give you an Overdue warning and then also give you the number of days overdue:
If(IsNull([Complete Date] and Today()>=[Due Date], "Overdue" & Days(Today()-[Due Date]),
If([Complete Date]>[Due Date], "Overdue" & Days([Complete Date]-[Due Date]), "On Time"))
This essentially just checks to see if no complete date has been entered and if the current date at the time of running the formula is greater then the Due Date it will display the Overdue message and the number of days overdue. If Complete Date has been filled in and is greater then the Due Date it will capture how many days overdue it is. Finally if it isn't overdue it will display as "On Time".
It might require some tweaking if you wanted the output to be adjusted or if your fields need to be matched up for type but this should be a basic framework to work from with room for customization.