How can it be reliable if scientists consistently make the earth older and older (and I don't mean one year at a time). Scietists have said the Earth was millions of years old and not its billions. Next century, it'll probably be trillions.
Try again. Scientific estimates of the age of the earth stabilised about 50 years ago (at around the time that a technique for determining the age of the earth by a reliable method emerged, by a curious coincidence) and haven't moved significantly since. Assessments of the age of the earth before the middle of the last century were little more than informed speculation, except that a lower bound could be put on the age by observing the rate of geological processes.
"Scietists have said the Earth was millions of years old and not its billions. Next century, it'll probably be trillions."
The estimate of millions of years old predates radioisotope dating--hell, it predates the revolution in physics wrought by understanding the structure of the atom which makes radioisotope dating techniques possible.