You can set the date on the left and the time on the right:
to change the year, click on the little arrows on each side of the year;
this updates the month view where you can click on the current day in order to highlight it;
it is recommended that you check that the time-zone settings are correct for your physical location. Click on the button and select the correct place in the tree view.
When you have chosen the timezone, a dialog appears asking whether your hardware clock is set to GMT. Answer if only GNU/Linux is installed on your machine, otherwise.
to change the time, you can then either move the hour, minute and second hands of the analog clock, or change the numbers below it.
When you are finished, click to apply your settings or to close the tool, which will consequently discard your changes. If you want to return to your previous settings, click on .