Final answer:
The precise duration for keeping a daily log by the carrier varies based on DOT regulations and airline policies, especially regarding service animals' documentation, which may need to be submitted 48 hours in advance. For personal expenses, it is advisable to track all receipts for one week.
Step-by-step explanation:
The length of time a daily log must be kept by the carrier depends on the specific regulations set by the Department of Transportation (DOT) and any additional requirements that airlines may have. DOT has developed several forms to attest to an animal's health, behavior, and training, which can be relevant when passengers travel with service animals. Airlines might require these forms be submitted up to 48 hours prior to travel or have them ready at the departure gate.
For general expenditures unrelated to service animals, tracking spending through the collection of receipts or noting expenditures in a smartphone for a week can provide insight into how much money is expended on 'Merchants of Cool' (non-essential items) and necessary transportation services.