Lots of posts on this, but to briefly recap, the membership year end date has nothing to do with your miles.

Your miles will expire after 3 years (36 month) of non-activity IIRC. Look at the last miles transaction on your account, you have 3 years from this date until your miles expire.

To extend the deadline, simply complete one miles transaction (i.e. spend or earn) for however many miles and it will extend the three years from that date. Basically as long as you do one transaction every three years, your miles will never expire.

Cheers
PhilH