T can be understood as the day of the transaction. For example, t0 and t 1 refer to the arrival time of the POS machine after swiping the card, t0 refers to the arrival time of the day after swiping the card, and t 1 refers to the arrival time of the second working day after swiping the card, so t4 refers to the arrival time of the fourth working day after the transaction.

The arrival time of credit card is uncertain, because the arrival time of each bank is different, and the arrival time of different POS machines may be different. Sometimes the credit card arrives in real time, and sometimes it won't arrive until the next day. The bank doesn't check out because of public holidays and weekends. So if you swipe your card from Friday to Sunday, you may have to wait until the bank goes to work on Monday to check out.