In today's society, we're slaves to the clock (remember the kerfuffle caused by the shift in daylight-saving time?). Our ancestors, on the other hand, were far more flexible about timekeeping. The average villager in the Middle Ages, for example, had no idea what year it was — it simply wasn't relevant to his life. Even centuries later, governments and churches changed calendars to move holy days and mark political revolution. In our forebears' world, the date might've been different in places just a hundred miles apart.

Naturally, this causes all kinds of headaches for family historians. How are you to record your ancestor's age if the birth date noted by the church clerk was based on their calendar, not yours? Or suppose you find a letter that says Great-grandpa Fred died “last Saturday,” or you have Great-aunt Edna's marriage year and age, but no birth date.