From then to now, many thousands of dates by many methods. When a crystal of a mineral forms it starts with some amount of a radioactive element. The decay rates of the element must be known. Then the ratio of daughter to parent can be used to calculate the age of the formation of the crystal. This has allowed fantastically detailed knowledge about the past both in terms of relative order and in terms of absolute time.
Lets look at the time scale as it stands now.
I know its difficult to remember, but here are some acronyms that will help you. Drawing of the outcrop on left, emphasizing layering. Sandstone must be younger than basalt. Dinosaur in sands which look like dune sands, in detail - probably are fossil dunes produced in a desert. Sands contain blocks of basalt arranged as talus cones - probably are fossil talus cones.