If terms are net 30 when does the invoice become 1 day past due?

If the invoice is terms of net 30, the invoice is past due on the 31st day after the invoice date. If the invoice is dated January 1st January, generally the date it's due by is January 30th, (you count the invoice date of January 1st as the first day), meaning one day past due would be on the 31st day after the invoice date, making it January 31st.