815 ILCS 205/5.1

Notwithstanding any other law to the contrary, it shall be unlawful for any person to charge interest on any first class claim as defined in Section 18-10 of the Probate Act of 1975 until 60 days after issuance of letters of office to the representative of the decedent's estate, or if no such letters of office are issued, then beginning 60 days after those amounts are due.