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?