Comment 1 for bug 869974

~/.xsession-errors is only created fresh by gdm and lightdm, e. g. kdm just appends (at least last time I checked). I think I'll better ask ConsoleKit:

 unix-user = '1000'
 realname = 'Martin Pitt'
 seat = 'Seat1'
 session-type = ''
 active = TRUE
 x11-display = ':0'
 x11-display-device = '/dev/tty7'
 display-device = ''
 remote-host-name = ''
 is-local = TRUE
 on-since = '2011-10-07T09:46:35.438201Z'
 login-session-id = '4294967295'

and take the on-since property.