By setting the the parameter HISTTIMEFORMAT in .bashrc script under a linux user, timestamp can be appended to each command executed by the user in .bash_history file.
Add the below line in .bashrc file of a OS user.
export HISTTIMEFORMAT="%Y-%m-%d %H:%M:%S "
Sample command history output
1002 2014-08-06 22:41:56 hello
1003 2014-08-06 22:41:57 history
1004 2014-08-06 22:42:04 who am i
1005 2014-08-06 22:42:09 who
1006 2014-08-06 22:42:11 who am
1007 2014-08-06 22:42:13 who am i
1008 2014-08-06 22:42:16 whoami
1009 2014-08-06 22:42:21 history
Hope this helps to some linux geeks.