Though the book perhaps gets a little too math-ey by the end (and this is coming from someone who is literally teaching discrete mathematics to university students at the moment), Dantzig provides a compelling overview of what we mean when we talk about "numbers," and why this is a question that we must repeatedly keep re-visiting.
It says how long ago something was said, so you can tell last month
from 2014 without doing the arithmetic. Full and green is recent; it
drains and cools as the date gets older, and empties completely at about
6 years.
5d5 days ago
8mo8 months ago
2yr2 years ago
4yr4 years ago
9yr9 years ago
A striped bar means the date itself is an estimate — the source did
not carry an exact one. An entry with no date at all gets no bar.