I'm interested in D&D 5e but I will also accept answers from previous D&D versions as well as similar systems (e.g., Pathfinder 1e/2e).
Janathiel II, famous historian, cartographer, and the Grand Wizard of Whitescar is studying the ancient structures of Ær-Toril known as Rhas.
These are gigantic and ancient structures/areas that cannot possibly have been created by nature. For example, Rhas Nolh is an almost perfect mountain pass that spans more than 100 kilometres, Rhas Aldhaen is an ancient forest that does not age (i.e., each tree is forever the same), etc.
Janathiel II has a theory: All of the Rhas constructs were created approximately during the same time period/era possibly by a technologically/magically advanced civilization.
To test this theory, however, he needs an accurate way of measuring how old an object is. Even worse, this method should be accurate even when counting thousands of years (if not hundreds of thousands).
If Janathiel II was living on Earth during the 21st century he could have used radiocarbon dating, but unfortunately for him, he lives in Ær-Toril.
Is there a spell, magical item, or any other method Janathiel II can use to accurately calculate how long ago an object/construct was created?
I am aware that I can create such a spell or item myself. Before I do that however I am interested in existing approaches, as well, some out of the box thinking.