With EOM terms, the payment clock starts on the last day of the invoice month rather than the invoice date. "30 days EOM" means 30 days after the end of that month.
EOM terms are common with larger companies that pay suppliers in a monthly batch. They can add almost a month to your wait if you invoice early in the month — so time your invoices accordingly.
Example
An invoice dated 3 March on 30 days EOM is due on 30 April (30 days after 31 March).