Well, in "A Book About Hype", one of the examples involves displaying the date. (I just loaded it up and forgot exactly what I did. It was cool to see a functional clock – in a book!)
The problem is that the book uses seconds, minutes and hours. That's not quite the easiest way to get a percentage. Instead, you'd probably want to use a "Timestamp". Basically, a Unix Timestamp is the number of seconds since January 1, 1970.
If you keep track of the seconds, then it's just a matter of calculating the percentage of current seconds vs the total available seconds. So, sounds possible.