John is buying a new car. The value of one car he is considering is $20,000. The value of the car as it ages can be modeled by the function V=20,000 (0.84)t, where t is the number of years from the time of purchase. One of the factors John is using to make his decision is the value of the car over time until it reaches half its original value. What values of domain are reasonable for the given function in this context?