It's really a marketing thing, not an expiration date. It's absolutely safe to eat a Clif Bar after the date on the package has passed. You just need to take a few seconds to look it over before you eat it.

Is it OK to eat expired Clif Bar?

Officially, they have a shelf life of 1 year from the date of purchase. However, extensive testing of Clif bars has shown that Clif bars are still edible more than 2 years after the best before date on the packaging if they have been stored away from heat and their wrappers are intact.

Do bars expire?

If opened, but still stored properly, the rule of thumb is one year. As for milk and white chocolate bars, the time available is cut in half. One year if unopened and stored properly, and 6-8 months if opened and stored properly.