It is stated in the Immigration Rules - 34G(3)
https://www.gov.uk/guidance/immigration ... -in-the-uk
Date an application (or variation of an application) for leave to remain is made
34G. For the purposes of these rules, the date on which an application (or a variation of application in accordance with paragraph 34E is made is:
(1) where the paper application form is sent by post by Royal Mail, whether or not accompanied by a fee waiver request form, the date of posting as shown on the tracking information provided by Royal Mail or, if not tracked, by the postmark date on the envelope; or
(2) where the paper application form is sent by courier, or other postal services provider, the date on which it is delivered to the Home Office; or
(3) where the application is made via the online application process, and there is no request for a fee waiver, the date on which the online application is submitted; or
(4) where the online application includes a request for a fee waiver, the date on which the online request for a fee waiver is submitted, as long as the completed application for leave to remain is submitted within 10 days of the receipt of the decision on the fee waiver application.
(5) Notice of invalidity under paragraph 34G(4) will be given in writing and served in accordance with Appendix SN of these Rules.
