After all these discouraging posts (including mine) about the [exact] dating of songs, let me put before you a totally different point of view:
I always tried to learn something about everyman's experiences in life when singing folksongs. There are a lot of themes, e.g. love where the experiences don't change over the millennia. The same feelings are expressed in a Greek poem of 700 B.C. as in a song of mid 18th century.

A very personal experience: When working on a construction site we used wooden scaffolds tied together with ropes There is an old song about the dangers of a bricklayer's job on such a scaffold and falling to death. When I heard it first I remembered my work, and I didn't give a dime for the time of its origin - such accidents could have happened from antiquity until the eighties, when they went over to better scaffolds of metal, screwed together, and with a lot of safety devices. So I can say I shared a dangerous work with people gone for centuries, and this was of greater value for me than the age of this song.

But be consoled: there are some songs about historical events which are easy to date (I just remember some famous songs from Germany): The first use of heavy artillery by Emperor Maximilian shortly after 1400 A.D., the battle of Pavia in 1525, the storming of Belgrad by Prince Eugene, and the battle of Sedan in 1870.