Extension COS on expiry of current COS or current visa

Hello All,

Good day!
I have got a strange question here.
Current visa expiry date : 26th Feb
Current COS end date : 11th Feb

New COS for extension should start from the next date of expiry of COS or VISA?

Anyways I will be able to make my application before both, only thing is what to mention in COS as start date?

Thanks and Regards
Saravana