thank you @Niyas Raphy , will check the link to learn further. regards

@Niyas Raphy, i just tried your code as below but when upgrading my app/module, it is saying that booking_date field is not in database ( i have used in patient_view.xml ).

booking_date = fields.Date.today() + relativedelta(days=3)

how i can have the field with data or this code is used in a compute function? or something else? i searched odoo default addons folder for relativedelta yet failed to find how to set default value as mentioned in my opening post above. or i am missing something or this is my wrong approach etc.?

please help.
regards

when using relativedelta.relativedelta(days=3) it is not starting server and showing error

type object 'relativedelta' has no attribute 'relativedelta'

may be i have to import something for this purpose.
regards

for field booking_date which type is Date is working fine now with code below:

booking_date = fields.Date(default=str(datetime.now() + relativedelta.relativedelta(days=3)))

but for Datetime type field it is thowing error when going to create New record. error as below (the last line only because there are several lines):

ValueError: unconverted data remains: .191905

may be i have to use some other pythong function to convert it correctly for Datetime field, please help.
regards

@Niyas Raphy thank you for helping us.

its done now, also modified Datetime type field to have default date + time as below:

appointment_time = fields.Datetime(default=datetime.today() + relativedelta.relativedelta(days=3, hours=3))

please advise if i am doing wrong or missing something.
regards