Devil, lightyears is a measure of distance, not time. It's the distance that light travels in 1 year.

MooK, in concept that'd work, except look at it this way: Say in the year 2000 the mirror is sent out. It'll take longer than a year to get it a lightyear away from us, so it's pretty much useless, because by the time it gets set up, we're only looking as far back as when we first sent out the mirror, or later.