early application but extension from expiry date - Possible?

Hello,

I would like to know how early can one apply for Tier 1 Extension to get extension from the expiry date. My current visa expires on 22nd March and need to get extended.

I heard that if applied not earlier than 28 days before the extension then extension is granted for 3 years from the expiry date. But I could not find any formal documentation about this.
I will appreciate you can guide me on this.

Thanks,
A