I always thought that being 30 days late meant that you were 30 days late from the due date.  

The grace period is built in and therefore not included.  So if your due date is today and you pay today (or tomorrow or whenever today's payment processes) then you are not 30 days late.  30 days late would be 30 days from now.

But you might get a late fee tho.  But not reported to your CRs as 30 days late since you are not 30 days late.