2.3.1 Starting with rocks
The time pertinent to our study of life is Earth time is geological time.  Our reading of the geologic clocks by their residual radioactivity still tends to miss by a few millions, or tens of millions, of years but the right orders of magnitude are known.
Among the relatively good readings are some of rocks that are nearly 2,000,000,000 years old. There appear to be rocks somewhat older than these but not yet dated. It is also probable that the earth existed as a planet before the formation of any of the rocks now exposed in its crust. From these data it is known that the age of the earth is more than 2,000,000,000 years. There is less direct and less conclusive evidence that this age is on the order of 3,000,000,000 years. This figure is staggeringly large, and yet it is finite and sets an end to wandering in the apparently infinite range of cosmic time. The possible range of time for life on the earth probably extends little beyond the established rock ages. The earth has been more or less as we know it, a fit abode for life, for a period on the order of 2,000,000,000 years.
The succession of events in earth history has been established much more accurately than their dates and durations in years. It is more useful and customary  to say that a given animal lived, for instance, in the Permian period than to say that it lived perhaps 215,000,000 years ago. We know absolutely that a Permian animal lived after its Pennsylvanian forebears and before its Triassic successors, even though we may have serious doubts just how many years, before and after, were involved. In discussion of the course of evolution some of these geologic period and epoch names must be used. Those necessary for this account are given in the following table. Besides the 2,000,000,000-year round figure for the oldest dated rocks, a few, but only a very few, fairly accurate year dates have been obtained corresponding with other parts of the geologic table of periods and epochs.
There is, for instance, a good radioactive clock reading of about 60,000,000 years for a rock in Colorado that belongs around the end of the Paleocene or beginning of the Eocene in the epoch table. In this and other ways it is possible to arrive at very rough estimates for the year ages and durations of all the various periods and epochs. Estimates by good authorities still commonly differ by as much as two to one for year durations of some of the periods, but discrepancies in well-grounded recent estimates are usually less than that. Rough as these approximations are in the present state of knowledge, they will prove useful in giving some idea of the durations of various events and the rates of some of the processes of the evolution of life.

The geological record has been used to set up a series of meeting places with our ancestral great grandmothers of the past.