How long is powdered tissue media good for?

[John Brunstein]

	Just wondering if anyone out there has any experience with using
tissue culture media (as made from the premix packages) after its 'expiry
date'....we seem to have a lot of DMEM premix still in the sealed foil
packets which has sat in our cold room for longer than 1 year.  Does it
really go bad or is this just a marketing ploy to encourage labs to keep
buying fresh supplies?