Work start date- Tier 2 General extension

Hi dear valuable members,

My Tier2 General visa is expiring this July 2021. I am trying to apply for an extension in the next month.

* Some one has mentioned that I can apply for extension any time and not just wait for 3 months before the visa expiry date. Can Moderators/senior members confirm this please?

* What work start date should be mentioned in the COS? IHS fee depends on the work start and end dates. Can I mention 15-07-2021 as my work start date even though it is so far from now ?

Visa Start date - 11-09-2011
Visa end date - 14-07-2021

I am a bit confused atm and couldn't understand what to request from my HR.
Any guidance would be very helpful.

Regards
Ramya

Sorry, my visa start date was mentioned wrongly in the above post.

Visa Start date - 11-09-2017
Visa end date - 14-07-2021