using expressed milk, how, when or waisted

Nicola • My husband & I together 16yrs, married 5yr. Poppy born oct 2016, 4months off the pill, diagnosed reflux, breast fed baby
I've just realised that if I plan to continue Breast feeding my stash of frozen milk will be waisted as it only has a 6 month seek by date.
I explained to my hubby I was doing it so that when I stopped or go back to work in Feb 2018   I could use my stash for my LO And this would extend the lengnth of time she has Breast milk. 
But I don't think this is reality????
What should I do, how can I use my stash for her or will I be better donating it?
Also what is the purpose/ how can I use expressed milk for my babies advantage please? I.e. Like I thought I could when explaining it to my hubby.