For example:
Day 1-receipt of a proper invoice
Day 30-required payment date
Day 45-end of grace period, no interest due
Day 50-payment date; interest due from Day 31 through Day 50
Day 80-final due date for payment of interest penalty
Day 81-additional interest begins on interest payment due.
4 Pa. Code § 2.35
This section cited in 4 Pa. Code § 2.37 (relating to responsibilities of Commonwealth agencies); and 4 Pa. Code § 2.38 (relating to exclusions).