A random fact I picked up from growing up with chickens and always having tons of eggs:
If you have some eggs and theyre past their best-by date, or you just arent sure how old they are, take a coffee mug and fill it with plain water, and drop the egg, gently, fat end up, into the water. If it sinks and stays sunken, its still edible/good. If it floats fat end up, its old and I wouldnt reccommend using it, but y'know. If it floats from any other side of the egg, throw it out, its bad.
The logic: Eggs have an air pouch on the fat end of the egg, held in place via Egg Physics (disclaimer: im a farm boy not an egg science boy), which is intended to provide the growing embryo with transfer of gases, via the semi-permiable membrane of the eggshell. As the egg gets older, the liquid in the egg will begin to evaporate, and that happens roughly in time with the amount of time an egg is "good" for. If for whatever reason the air pouch gets broken or moves around in the eggshell, outside of intentional cooking methods, it can quicken the going bad process or indicate something is wrong with the internal membrane (the "skin" you sometimes see on hardboiled eggs when peeling em) and generally you dont wanna risk eatin em like that.
Generally, youre gonna get the longest shelf life out of fresh eggs/local eggs, and i've found that brown eggs tend to last longer (and be better in every way, call me a snob) than white eggs, which often have the waxy layer on the shell scrubbed off during processing, which effects how well they last. But since not everyone has the privilige of options in these sorts of things, the float test is a good way to test your eggs, because the refrigerated shelf life of even walmart brand white eggs is longer than the printed use by most of the time, and even room-temperature, its considerable.
TL/DR: Eggs have air in em and the older they are the more air they'll have. If there's too much air in the eggs, theyll float in water, and are probably too old to eat.
