The starting isotope is called the parent and the end-product is called the daughter.
The method of calculating radiometric dates is like using an hourglass.
You can use the hourglass to tell time if you know several things: the amount of sand in the top of the hourglass when it started flowing, the rate that the sand flows through the hole in the middle, and that the quantity of sand in each chamber has not been tampered with.
Knowing this first: that scoffers will come in the last days, walking according to their own lusts, and saying, “Where is the promise of His coming?
For since the fathers fell asleep, all things continue as they were from the beginning of creation.” For this they willfully forget: that by the word of God the heavens were of old, and the earth standing out of water and in the water, by which the world that then existed perished, being flooded with water.
However, there are many methods that can be used to determine the age of the earth or other objects.