Você pode usar o campo lstart
para isso:
# ps -aefO lstart | head
PID STARTED S TTY TIME COMMAND
31379 Sat Aug 2 21:31:11 2014 S pts/3 00:00:00 zsh
31399 Sat Aug 2 21:31:11 2014 S pts/3 00:00:00 \_ vim
27313 Sat Aug 30 13:19:27 2014 S pts/4 00:00:20 zsh
27193 Sat Aug 30 13:18:11 2014 S pts/2 00:00:19 zsh
25196 Wed Sep 3 22:37:57 2014 S pts/16 00:00:00 zsh
25231 Wed Sep 3 22:38:00 2014 S pts/16 00:00:00 \_ sudo
25241 Wed Sep 3 22:38:04 2014 S pts/16 00:00:02 \_ zsh
24642 Sat Aug 30 12:50:48 2014 S pts/0 00:00:20 zsh
23366 Thu Sep 4 08:47:30 2014 S pts/18 00:00:00 zsh