It is not possible to move the time backwards as this would affect all the history data. If you try to reset the time to an earlier time than the last recorded data then the setup process will not allow this, it will keep looping around the time-month-day-year set up process. You cannot exit this until a time is entered that is later than the last recorded data.
The only way to move the time backwards is to clear the historical data from the unit by pressing and holding the 'History/ Clear Mem' button for 5 seconds. After this is done you can then go back into the menu setup and adjust the time.
This does not affect moving time forwards.