 The clocks were designed to capture the link between an aspect of our biology, DNA methylation, and our age. But what happens if the relationship between those two changes? For example, lifestyle choices or diseases can make biological markers like DNA methylation look more like those in an older person. In that case, an epigenetic clock would report that the person is older than the number of years since they were born. That mismatch is the difference between biological age, as measured by epigenetic clocks or other biomarkers, and chronological age, which is simply the number of years that you've been alive.