HISTTIMEFORMAT="%d/%m/%y %H:%M "
For any commands typed prior to this, it will not help since they will just get a default time of when you turned history on, but it will log the time of any further commands after this.
If you want it to log history for permanent, you should put the following
line in your ~/.bashrc
export HISTTIMEFORMAT="%d/%m/%y %H:%M "