The good dates are confirmed using at least two different methods, ideally involving multiple independent labs for each method to cross-check results. Sometimes only one method is possible, reducing the confidence researchers have in the results. And ugly dates?

Is biostratigraphy absolute or relative?

Biostratigraphy is the process of using the fossil organism assemblages in the rocks to determine their ages, a form of relative dating. Because fossils evolved slowly through time, the presence or absence of certain fossils, called indicator taxa, can tell a geologist what time period they are looking at.