Formula milk

Hi Mummies, during the 1st month after I gave birth I have been supplementing baby with formula in addition to breast milk due to insufficient supply. Since 3 months ago, I started to fully breastfeed. My question is, the formula which has been opened since mid Feb, is it still safe for baby to consume as the guide is to consume within a month? I’m using enfamil stage 1 and there is still half tin left. I may need to supplement again as baby’s demand increases these days. Thanks in advance for your advice.