The half-live of potassium-40 is approximately 1.26 billion years (that is, 1.26x10 years).
Obviously, this formula depends on the laws of physics remaining constant over time.
The method is based on the fact that the potassium-40 isotope of potassium decays over time to form argon-40.However, some creationists have argued that God increased the rate of potassium-argon decay during the first few days of Creation, thus causing the potassium-argon dating method to give erroneously old date readings. But after the rock solidifies, any potassium-40 that is present continues to decay, and the argon-40 that is produced cannot escape from the rock.Thus, geologists use potassium-argon dating to measure the age of volcanic rocks.
If the concentration of argon-40 is almost zero, then the rock was formed recently.If it is high relative to the amount of potassium-40 present, then the rock is old.