the payment was there on time and it didnt get posted

Quoting REAL PERSON
I wanted to see if you can help me. One of my junior loans posted a 30 day late and brought my score down. i'm now in the lows 600's however, the payment was there on time and it didnt get posted because the last two digits of the account numbers were wrong. what should i do? i contact washington mutual in which i have my loan from and they don't want to take it off my credit report. they show the payment was there on time, but didnt post because of the account number.
REAL PERSON
---------------
that is thier mistake if you can show it cleared your bank on time
they should remove it due to thier error
evidently they must think it is not
30 days is thew cutoff and that IS a long way from on time
but the letter of the law says they cannot report you late until 30 days.