I tried export
HISTTIMEFORMAT='%D %T' but it seems the time stamp of commands executed before I set HISTTIMEFORMAT will be the time I logged in current session. Is this normal? Is there any way to check when did the previous command executed? Thanks.
That's normal. If
HISTTIMEFORMAT is set then the times are stored in history. If they're not stored then it has nothing to go on except the start time of the current shell.