American Express tells online customers that they’re 30-plus days past due when they’ve only missed their payment deadline by a few days, according to a study released Monday on late-payment policies of credit card companies.

“This may cause stress for those who know that most credit-card companies will report them to the credit bureaus once they become 30-plus days past due,” said Odysseas Papadimitriou, chief executive of CardHub.com, a credit card research Web site.

CardHub.com decided to investigate the late payment policies of major credit card issuers after an experience that he had with American Express. He recently realized he was 12 days past due in making a payment for his American Express credit card bill. When he went to pay his bill online, he received a message informing him that he was 30+ days past due.

Although American Express says it’s a “nomenclature” issue, the phrase “past due” is not an ambiguous term, CardHub.com said. “‘Past due’ means past the due date for payment, not past the time a bill was generated,” it said.

For its part, American Express said the confusion stems from the fact that it uses a “different nomenclature for its internal aging than other issuers.”

But “we are consistent with the industry with applying Penalty APRs when a cardmember is delinquent,” a spokeswoman said.

“In compliance with the card act, we do not apply a Penalty APR to existing balances unless the cardmember failed to pay within 60 days of the payment due date,” she said in a statement.

“Regarding the screen shot of the American Express bill online that said ‘30 Days Past Due,’ this refers to the aging of the account and does not mean that the account is 30 days delinquent,” American Express said. “We are presently working on making this language more clear for customers. For example, saying something like “This account is past due” or “This account is seriously past due.”

To see the CardHub.com study, click here.