Hi Cybrosys. Thanks for your reply.

But I found a workaround for existing vendor bills.

1. Reset posted Vendor Bill to draft.
2. Show Start Date and End Date columns in the list view column selector (if not yet visible).
3. IMPORTANT: Vendor Bill must be set as "Bill" type first (if previously set as "Receipt" type). This way entering values in Start Date and End Date columns will be accepted (usually appears in different color yellow when acceptable).
   
4. Enter the values for the Start and End Dates as normally.
5. When you confirm the Vendor Bill, the Deferred Entries smart button will appear containing the auto-generated journal entries.
6. WARNING: I don't know if this is a bug, but do not save the vendor bill as "Receipt" type when generating the deferred entries! I noticed that the deferred entries will instead be put under the Journal you specified for Deferred Revenue in the Settings. This is true when I tested by editing an existing Receipt type of Vendor Bill or creating a new one. Unfortunately, if you switch back to "Receipt" type of Vendor Bill after deferred entries are generated, they will change again as deferred revenue entries.

Hope anyone else who encounters this finds this helpful.

Hi,

In Odoo, deferred expense fields (Start and End Dates) are only editable when a vendor bill is in draft. For previously posted bills, even if you reset them to draft, Odoo often prevents saving these fields because deferred entries are only generated when the bill is first confirmed with the dates present. This restriction exists to avoid inconsistencies with already posted accounting entries.

The recommended workaround is to duplicate the original vendor bill, set the Start and End Dates on the new draft bill, and then confirm it. This ensures that deferred entries are generated automatically, while the original bill can be canceled or archived if needed.

If duplicating is not possible, such as when the bill has already been paid, you can manually create deferred journal entries by calculating the monthly expense portion and posting entries from a Deferred Expense account to the actual Expense account over time. For bulk adjustments, a programmatic approach using a server action or script can be used to recreate draft bills with deferred dates for automatic entry generation.

Hope it helps