if you just bought it i would take it back.
the other important thing to look at besides the date is if it says "best by," "use by," or "expiration date." "best by" means that item is best by that date. it's unlikely to kill you after that date, but it might not be at its freshest. think chips. if you have ever had chips a little out of date they are stale, but not likely to cause you to come to harm. most waters i have seen have "best by" on them. personally i have had water about 2 years past the "best by" date, but that was also plain water. i lived and was fine.
"use by" or "expires on" tend to be a little more end of life for the product. less than three weeks out i wouldn't worry too much if i already had it, but if i am considering ingesting something like that i tend to go by what it is, taste, smell, exactly what it is and other obvious signs of it being okay. and i tend to err on the side of toss it for those two options.