You searched for: “glottochronology
1. The application of statistics to vocabulary to determine the degree of relationship between two or more languages and the time of their splitting off from a common ancestor.
2. The determination of how long ago different languages evolved from a common source language.

The branch of lexicostatistics that studies the rate of the replacement of vocabulary and attempts to determine what percentage of basic vocabulary two presently distinct, but related languages share, using the information that is obtained to estimate how long ago they ceased being a single language.