The article outlines the steps to add a Closing Date.
FOLLOW STEPS BELOW:
1. Open the Admin Portal.
2. Click Lookups and Scroll to Closing Dates and click.
3. Click the green + button to add a Closing Date.
NB. For a quick tutorial watch below

The following rules apply:
A new closing date cannot be equal to or earlier than an existing closing date. Thus, it won’t be possible to pre-emptively capture closing dates for each month.
Once you have set a closing date you cannot backdate Finished Goods or Raw Material stock movements.
Once you have set a closing date you cannot backdate Credit Notes or invoices into the closed period.
Once you have set a closing date you cannot do a landed cost update for items received in a closed period.
You will not be able to set a closing date if there are material stock items GRV'd prior to the closed date but not yet marked as Checked.