Press Ctrl+Alt+T to open a terminal, then run one of the commands below:

HISTTIMEFORMAT="%d/%m/%y %T "  # for e.g. “29/02/99 23:59:59”
HISTTIMEFORMAT="%F %T "        # for e.g. “1999-02-29 23:59:59”

To make the change permanent for the current user run:

echo 'HISTTIMEFORMAT="%d/%m/%y %T "' >> ~/.bashrc  # or respectively
echo 'HISTTIMEFORMAT="%F %T "' >> ~/.bashrc
source ~/.bashrc

To test the effects run:

history

For commands that were run before HISTTIMEFORMAT was set, the current time will be saved as the timestamp. Commands run after HISTTIMEFORMAT was set will have the proper timestamp saved.