It rubs me the wrong way when most people refer to best by date on food items as the expiration date. Looks, smell and taste go far in deciding if food is good to eat or not.
Interestingly even medicines don’t magically go bad the day after the expiration date.
It's also a logically trivial statement. There is no threshold for "best"--food quality is a matter of degree. So you can cite any date at all and justifiably say the food is "best" before that date.
Some medicinal compounds are hygroscopic and may go bad over time due to the permeability of moisture through the packaging. (Hence the use of metallized foils, which have nearly zero moisture permeability). Same for oxygen/oxidisation.
Interestingly even medicines don’t magically go bad the day after the expiration date.