Summary: A layman reading of Home Office guidance / Immigration Rules indicates that you will receive the pre-settled status from the date of approval should the application be successful. However, for the purpose of qualifying for the settled status, the time of continuous residence begins from the date when you first arrived in the UK, assuming no gaps are created.
In a statement of intent, the Home Office mentioned the following regarding the EU settlement scheme:
3.5. Pre-settled status means that the person will (in all cases) be granted five years’ limited leave to remain, and they will be eligible to apply for settled status (indefinite leave to remain) as soon as they have completed five years’ continuous residence in the UK (less in some particular circumstances: see paragraph 3.7, below) ...
This is then implemented in Appendix EU of Immigration rules, which says:
EU3. The applicant will be granted five years’ limited leave to enter (where the application is made outside the UK) or five years’ limited leave to remain (where the application is made within the UK) where:
- A valid application has been made in accordance with paragraph EU9;
- The applicant does not meet the eligibility requirements for indefinite leave to enter or remain in accordance with paragraph EU11 or EU12, but meets the eligibility requirements for limited leave to enter or remain in accordance with paragraph EU14; and ...
The ways the statement and rules are written suggests that you are granted five years of leave to remain (colloquially referred to pre-settled status) upon a successful application, not when you entered the UK.