Define absolute dating geology
1.5 and 1.6 of Harland ) and a few numerical estimates were available previously (but often for the duration of the entire scale rather than its individual subdivisions).
In addition, like any good scientific measurement, every dated boundary has an uncertainty associated with it, expressed as " - X millions of years".
What these people were seeking was a numerical date of Earths age.
Numerical dates pinpoint the time in history when an event took place, such as the extinction of the dinosaurs 65 million years ago.
These can not be included in the diagram for practical reasons, but can be found in Harland , 1990, along with a detailed description of the history of earlier-proposed time scales and the terminology, methodology and data involved in constructing this geological time scale.
Because of continual refinement, none of the values depicted in this diagram should be considered definitive, even though some have not changed significantly in a long time and are very well constrained (e.g., the Cretaceous/Tertiary boundary has been at 65 -1 Ma for decades, and has been tested innumerable times, with almost all dates somewhere between 64 and 66 million years).
Revisions to the relative time scale have occurred since the late 1700s.
Most scientists and many Christians believe that the radiometric dating methods prove that the earth is 4.5 billion years old.The overall duration and relative length of these large geologic intervals is unlikely to change much, but the precise numbers may "wiggle" a bit as a result of new data.Throughout the centuries mankind has striven to try to determine the exact age of Earth.The textbooks speak of the radiometric dating techniques, and the dates themselves, as factual information.Far from being data, these dates are actually interpretations of the data.