buy (something) on time
To buy something with the promise of paying for it later, as with a credit card as opposed to cash. I ran out of money at Christmastime and had to buy some presents on time, so I'm dreading my next credit card bill.
buy something on time
to buy something on credit. Tom: That's an awful fancy TV. Are you sure you can afford it? Jane: I'm buying it on time. I bought the sofa on time, but I paid cash for the chairs.