You've got to understand that the age in which we lived was quite a long time ago, long before your grandfather 'grandfather. You needn't know specific numbers; a general idea that it was many years ago is good enough.
所以只要明白我們生存的年代已十分久遠，就是在你祖父的祖父的…… 之前，可以不必詳記數字，只要有舊新之別的概念即可。