Tier 2 General Visa Extension date calculation

Hi All,

My current T2 General visa status is:

Work Date on COS:
Start date: 18 June 2018
End date: 18 June 2020

BRP Dates:
Date of Issue: 18-07-2018
Valid until: 02-07-2020

Queries:
1. What is my end date? Shall I refer BRP or COS?
2. As per my understanding I can apply for extension 3 months before End Date. what if I apply for extension in March? What date it will be having on new BRP? WIll it start from June or March?

Thanks,
Yoj

1. Visa/BRP end date.

2. New brp is valid from date of approval usually.