In a simple world, someone fills in a membership form, pays by credit card and their membership is active immediately.
In the direct debit world, things happen on different dates:
setup_date- fill in a membership form, complete a direct debit mandate
- Recurring Contribution created with status In Progress
start_date = charge_date
- Contribution created with status Pending
receive_date = charge_date
Membership created with status Pending
charge_date- first payment charged (4-5 working days later)
webhook_date- GoCardless fires webhook (one working day after the
Contribution updated to status Completed
Membership updated to status New
join_date = start_date = webhook_date
end_date = start_date + membership_length
This appears to be identical date behaviour to creating a membership with a pending cheque payment and then later recording the cheque as being received. The membership will start when the payment is recorded and the contribution status set to Completed. The end date is recalculated to be a year (or other membership length) from the start date so that members get a full year of benefit.
However... some might want the membership to start as soon as the mandate is set up, before waiting for the first payment. The current date logic is handled by core so this extension would need to override that in a couple of places to implement a different scheme. Since that is not specific to this payment processor, it might be better to do this as an enhancement to core, or a separate extension.
See also: How to set up for membership