Aging of a book or portion of a book based on the language or vernacular used by the author. Although a 30 year old book may be written about a time period 100 years into the future, you can still determine when it was written by looking at the vernacular used.
The GRE had several questions on baging of books.
I baged the book to be written in the early 1950's
What do you bage that book to be?