why does milk of magnesia have an expiration date? I noticed an old bottle of milk of magnesia in my refrigerator and saw that it had an expiration date printed: January 2015.  Checking the ingredients, I saw only three: magnesium hydroxide, purified water, sodium hypochlorite.  I understand that milk of magnesia is more or less a mineral solution.  How could this require an expiration date?  What would be the consequences of taking milk of magnesia past the expiration date?

(I see that this page discusses expiry dates on chemicals in the abstract; I don't know which of these arguments apply to milk of magnesia in particular, or any other over-the-counter dietary supplement or drug.  As an IT professional, I have absolutely no experience or expertise in the physical or life sciences.)
Many countries require anything marketed as edible to have an expiry date. You can speculate about what might spoil your Magnesia (see comments above), but likely it will be good four or fourty times as long as indicated. 

Of course the lid might not be perfectly tight, and it could dry up or get contaminated by bacteria, but this can happen anytime. And certainly after some time it will separate into clear water and a thick suspension of magnesia particles and you'll have to shake or stir it very thoroughly before consummation.

Anyway these expiry dates are 99% baloney. Very few things really get bad before you see the mould grow on them.

Fun fact and (german) exemption to that rule: After six months at the latest, the taste of beer (the non-pasteurised stuff) starts to deteriorate. Yet, according to the date on the bottle, you have 12 more weeks to drink the remainder of that crate with increasing disgust before it "expires".