Does magnesium chlorida have an expiration date?

All drugs, even supplements and other over the counter drugs, have an expiration date and a Control Number/Lot number. The expiration date is not an "Oh My God" date: it is just a date that the drug starts to break down and lose it's efficacy. I'd throw it out safely if it had reached that date.

The control/lot number allows the manufacturer to track batches of the drug to determine locations, if there is ever a problem that requires a recall. When filling a prescription in the pharmacy, typically pharmacists keep a record of control numbers also for such reasons.

So, yes, examine your bottle of Magnesium. It may be hard to find, but there should be one. Sometimes the date is imprinted on the bottle in exactly the same color as the bottle.