Download & Install: Install the Purchase Backdate and Postdate in Odoo from the Odoo App Store.

Navigate to the Purchase module, then go to Configuration >> Settings.
Scroll down to the Purchase Order Backdate section.

Once enabled, the following options will appear. Configure them to fit your workflow: Sync Bill Date with Purchase Order Date, Sync Receipt Date with Purchase Order Date, Enable Purchase Order Notes

When Enable Purchase Order Notes, then the Require Notes for Purchase Orders field will appear.

For Bulk Updates: To allow a user to backdate multiple orders, navigate to
Settings >> Users & Companies >> Users.

Select the user, and under theTechnical access rights, check the box for Mass Purchase BackDate Setter.

To Backdate a Single Purchase Order:

Navigate to the Purchase module and open or create a Request for Quotation (RFQ).

Click the Order Deadline (which serves as the PO date) and select your desired past or future date.
Fill in the Notes field if it appears and has been made mandatory in the settings.
Click Confirm Order. The date you chose is now the official Purchase Order Date.

Use the smart buttons to check the corresponding Receipt and Vendor Bill.
The dates on these documents will now match the new Order Date (if you have enabled this in the settings).

To Backdate Multiple Purchase Orders at Once:

Go to the main Request for quotation list view by navigating to
Purchase >> Orders >> Requests for Quotation.

Select the checkboxes next to all the RFQs or Purchase Orders you wish to update.
Click the Action button at the top of the list.
Choose the Multiple assign backdate option from the menu.

In the pop-up window that appears, set the new date and add any remarks. Click confirm to apply these changes to all selected orders.