How to find the start date of the current visa or leave to remain for a UK spouse visa renewal? [closed]

The form for renewing a UK spouse visa asks the question "What is the start date of your current visa or leave to remain?" (as of October 2019)

How should the applicant find the answer to this question?

There are multiple dates provided on various pieces of documentation associated with the initial approval of the spouse visa.

The most likely dates seem to be:

1. The "Valid from" date found on the UK entry clearance attached to the applicant's passport

2. The "Issue date" found on the biometric residence permit (BRP) card

3. The actual first date of entry to the UK (using the entry clearance with intention to immigrate)

In at least one applicant's case, the issue date on the BRP is earlier than the valid from date of the entry clearance (which is earlier than the date of entry). Information provided online by other applicants suggests that the dates are not always in that order.

Which date is "the start date" and where is the UK government's documentation that explains this?