Chronological bias simply refers to time as a potential confounder in research. Study recruitment can often take many years, and during those years medical treatment can change. That change in management over time is a confounder or bias in the the study, in that it is a variable that systematically affects the observed outcomes but is not part of the study design.
Chronological bias can also occur when disease definitions or categories change over time. For example, TIA used to be a purely clinical diagnosis, but in 2009 the definition was changed to include MRI criteria. That meant that the sickest TIA patients were reclassified and were now defined as having a stroke. The result is that both TIA patients and stroke patients look healthier after the definition was changed, and the results of any study that compared patients before and after that definition change would be biased.
This post is part of a series of posts on bias in medical research. You can find the whole bias catalogue here.