Length of Eternity

Studying Geology in U.C.G. eons ago (geologic time),I came across this wonderful evocation of eternity:

High up in the north, in the land called Svithjod, there stands a rock. It is a hundred miles high and a hundred miles wide. Once every thousand years a little bird comes to this rock to sharpen its beak. When the rock has thus been worn away, then a single day of eternity will have gone by.

(From The Story of Mankind by Hendrik Willem Van Loon)

I mention it because I really like it and secondly because elsewhere on the net this day is calculated, see http://www.maths.manchester.ac.uk/~cds/articles/svithjod.html