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Seems like it should be good for thousands of years, if not millions.

uhoh
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Mom's Basement
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    What kind of container is it in? – Karsten Apr 03 '22 at 01:01
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    The best before date often also serves as a lot number. So if there was e.g. chemical contamination at the factory, they could withdraw all packages with best before date between X and Y. – jpa Apr 03 '22 at 17:55
  • You would not want to buy salt package 1000 years old, would you? – Poutnik Apr 04 '22 at 04:51

2 Answers2

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This table (no pun intended) from Morton Salt explains why table salt has a shelf life:

MORTON SALT EXPIRATION GUIDE

Essentially, salt is hygroscopic, which means it absorbs water from the air. Thus, unless it is stored under very dry conditions, the salt granules will clump together over time.

This clumping is inhibited by anticaking agents. According to Morton, in the presence of an anticaking agent, table salt has a shelf life of ~5 years. I.e., under typical storage conditions, it should last that long before it begins to clump. Without an anticaking agent, it's ~3 years.

Interestingly, even if caking weren't an issue, if the salt is iodized, and you want it to remain iodized, it still has a shelf life of ~5 years. That's because, as the table explains, "iodine can dissipate over time."

Finally, as suggested by Karsten's comment, the cardboard container will also break down over time.

theorist
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    Culinary sea salt does not usually contain iodine or anti-caking agents. In the absence of iodization and preservation with antioxidants, the expiration date is merely a marketing gimmick. – AChem Apr 03 '22 at 01:41
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    @Farooq: As the link in this answer shows, culinary “sea salt” certainly can contain anti-caking agents. And as the answer explains, the (potential very slight) deterioration that justifies the “best before” date is not caused by the anti-caking agent, but inhibitied by it. Best before dates are never meant to be interpreted as “product expires now — discard it”, but just as “past this date, the manufacturer doesn’t promise that the quality will still be up to expectations” – Peter LeFanu Lumsdaine Apr 03 '22 at 17:41
  • slightly related: What component(s) of instant coffee make it deliquescent? – uhoh Apr 04 '22 at 02:31
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    The container is glass and plastic. If there was an anti caking chemical, shouldn’t it have been listed in the ingredients? Best by date was 2010. My personal experience is that, like wine, it gets better with age. – Mom's Basement Apr 05 '22 at 00:59
  • @Mom'sBasement (1) If an anti-caking agent isn't listed in the ingredients, and the manufacturer was accurate in their labeling, then there is none there. (2) Some plastics can release odors into what they contain over time. (3) Storage conditions can vary widely, but it's possible their estimate was too conservative—why don't you contact the manufacturer and ask them their rationale for their best-buy date? – theorist Apr 05 '22 at 01:37
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Salt does not expire. It does not decompose under normal conditions, and as you correctly assume, it could sit around for millions of years without undergoing any change. Sitting around for millions of years is actually exactly what it does as rock salt. However, while salt does not expire, other constituents of your salt package probably do; the expiration date is actually of the container itself.

Sea salt is different than table salt. It is less processed and does not have anticaking agents nor iodine compounds added to it. Also, while sodium chloride is hygroscopic, it is not that hygroscopic. It is rarely, if ever, used as a desiccating agent. Sea salt crystals tend to be larger and rougher than table salt particles and do not clump up as easily due to moisture. Therefore, iodine dissipation and bulking of salt particles are not as much of a concern as the other answer claims while ignoring that you explicitly specified "sea salt" instead of "table salt".

user122979
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    Try living on a boat with salt and ocean air and you will find that salt is VERY hygroscopic. Doesn't care if it has anti-caking agents, is granules or sea salt, the moisture in the air on the ocean gets into almost all containers. I use a snaplock glass container to get 6 months without clumping. But to use the salt you have to open the container and that is enough time for the salt to grab moisture from the air. – boatcoder Apr 03 '22 at 20:39
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    "It is rarely, if ever, used as a desiccating agent." See Dehumidify Your Home ¶ Also: "I placed salt in a glass, covered it with enough water and left it overnight. To my surprise, the volume of water increased by around 20-25%, the glass is significantly more full." — Scientific Reason for Salt Solution Gaining Volume. – Ray Butterworth Apr 04 '22 at 01:07
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    @boatcoder Being hygroscopic is usually assumed to be the property of material at usual room conditions, otherwise all soluble substances would be hygroscopic at 100% humidity. Table salt if being pure sodium chloride does not attract water until relative humidity > 75% ( T dependent ) – Poutnik Apr 04 '22 at 06:54