A credit card is a loan with a moving target. The user spends throughout the month; on statement day the running balance is FROZEN as a 'statement balance.' They have until the payment due date (typically 21 days after statement) to pay any portion, minimum (interest-bearing), partial (mixed), or full (no interest). What they pay determines what interest you accrue against them next cycle. Your books need three views of the same debt: current balance (live), statement balance (frozen at cycle close), and unpaid statement balance (what interest accrues against after due date). This lesson walks one cycle from purchase to interest accrual.