AllFinancialMatters lays down the math on why two-cycle billing sucks.

Companies using two-cycle billing, like the Discover credit card, calculate your average daily balance using two months.

The effect is that you can end up paying interest on a balance you already paid off; potentially disastrous for customers carrying large balances.

As ever and always, only use a credit card if you can pay it off in full every month. Adding to that, avoid the Discover card.

"It pays to Discover." Pays who? Not you. — BEN POPKEN

How Two-Cycle Billing Works [All Financial Matters]