In a separate article (Radiometric dating), we sketched in some technical detail how these dates are calculated using radiometric dating techniques.
Vast amounts of data overwhelmingly favor an old Earth.
Several hundred laboratories around the world are active in radiometric dating.
The simplest means is to repeat the analytical measurements in order to check for laboratory errors.
Another method is to make age measurements on several samples from the same rock unit.
In the particular case that Morris highlighted, the lava flow was unusual because it included numerous xenoliths (typically consisting of olivine, an iron-magnesium silicate material) that are foreign to the lava, having been carried from deep within the earth but not completely melted in the lava.