FLR FP Extension 28 days

Hi all
I am completing the application for FLR FP online and it says I can apply for extension before 28 days. Current visa expires on 17/03/2022.
Just wondering 28 days counts when the Biometric is completed or the application is submitted online? I don't want to delay and would like to book biometric as soon as possible. If it's online submission date, what's the earliest date I can submit and start uploading documents

Date of application is the date you submit and pay online. It is not the biometric appointment date.

17 Feb is 28 days before.

Thanks Char