How long is formula powder good for?

Purplehayzes

I EBF my daughter, but she will take a bottle now and again when needed.  I have 2 bottles of pumped milk in the freezer.  I'm leaving my daughter with her Grandma tomorrow night, and I'm not sure if the 2 bottles will be enough.  Anyway....

I have leftover formula from my son (who is now 2 yrs old).  It's a half-full can of opened formula.  The best before date on it says May 2013.  Is it still ok to use, even though it's been sitting in my cupboard for year?  If it's good, then I'll use it as back up to give my daughter while I'm out tomorrow night.

TIA for the advice