"x time ago" is unclear - it doesn't show when something was listened to, but how long ago relative to when it thinks "now" is
Whereas an exact time stamp/date tells me everything right away without having to start counting or thinking "oh so was that monday, or it was the tenth.. hmm.. so what did I do 3 hours ago...?" "

I'd much more prefer just the actual time of the listen, like "12:25:55" or whatever.
for example, 1st april 2010 12:30 or "18:10" are good - but "8 minutes ago" is bleh
at the very least an user option would be very good.