Formula Expiration

I'm primarily breastfeeding, but in order to keep up with demand we've been having to use some formula. We mix formula in her bottles while she's at the babysitter's 2:1. A friend of mine just told me though that a can of formula is good for only a month after it's been opened! I had no idea. I thought I could continue using it until the expiration date on the can.

Is it true that I can't use it after a month?

I guess I need to learn proper handling instructions for formula. Is there any other important stuff I should know?