The Science channel is actually running a very informative show on "Cosmic Time" over the next couple of days. Michio Kaku is the physicist ... visit the following link for show times...
http://science.discovery.com/tv-schedules/series.html?paid=48.15381.120811.33080.x
Actualy you've asked a very deep and probing question and to properly answer it would take far more time and space to provide this detailed answer. :-) A short description is that mathematically it is a dimension similar to a spatial dimension from Einstein. However it is parameterized...