I'd probably err on the side of caution with that formula tin, Neeny and Sim. Formula is inherently subject to bacterial colonisation, by the nature of the processes used to make it and its ingredients. It might be ok for one child, but that's no guarantee that it will be ok for another to take 'out of date'. Hence the rigorous sterilising that is required when using formula, for the Just In Case scenario. That's not my opinion, that's anything I've ever read about formula in baby books and on the web
You can get small sachets of formula for emergencies when BF'ing - not too cheap, but cheaper than buying a tin and discarding most of it because you have gone over the Use By date. One of the mummies in my Mothers Group orders hers online to get a slightly better deal, and she's the one who told me about it.
I've just noticed some red spots on Oscar, going to look up this Rosie Posie thing that some of your kiddies have had (wonder what Google would come up with if I typed that in!)...