If you already are in Period 12 and want to have a period 13 then you can, providing you haven't posted anything to the effective period 13 dates.
To add period 13:
Ask all users to log out of the database.
Click System, Options, System Options.
Click the Date and Periods tab.
At this point it is strongly suggested that you take a screenshot of this screen to remember the dates that will change and need to be replaced.
Ensuring that no other users are logged in you can change the No of Periods from 12 to 13.
When you change the period you will see that the periods range change. If you want to have a 1 day period 13 you will need to change the start date of periods 1 to 12 to what they were on the screenshot. Leaving the end date of period 12 one day short for period 13.
Running a period end on period 12 opens P13 for adjustments only. Running a year-end on period 12 opens period 1.