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