What, then, is time?
If no one ask of me, I know;
if I wish to explain to him who asks, I know not.
St Augustine, The Confessions, book XI
A scholarly - highly readable - article by Gregory M. Nixon on the role that consciousness may (or may not) play in the human perception of time.
Love of philosophy (would that be philosophiophilia?) required.
Because, let's face it, most of us here aren't really dreaming about mechanical time machines, are we? :-)