Hello!
To properly categorize this, you can use the multiple
IF function in excel, which is the IFS Function.
IFS Function works like this:
=ifs(argument 1, value if true 1, argument 2, value if true 2, ....)
so you can use this for multiple arguments. Regardless using days/months, this formula is ideal in categorizing your invoices. Also you can use =Today() so that Excel can automatically update what items are due and how many days are there remaining before an invoice is due.
Hope this helps!