Answer:
I do not recall the FCRA defining this period. I have seen reports made that counted the actual number of days and some reported that 2 payments owed was a 30-day account because a "month" often assumes 30 days and two payments is a 30-59 day account. The Consumer Data Industry Association (CDIA) is generally followed by the CRAs and it recommends a strict counting of the days, but CDIA is not the FCRA.
And yes, Payments due April 1 and May 1 would reach a 60 day status on May 31 using the strict counting of actual days.