What does the date on eggs mean if it does not say sell by or expires by in the United States?
In the US, the most important thing to look for on the carton is the number which shows the day of year that the eggs were packed. If the number is 1, that's January 1st. If it's 365, That's Dec 31st. In other countries the system may be different, so find out what system in your country indicates the pack date, if possible. Note: apparently some states in the US don't regulate this, so you would have to see what your state's protocol is, if not. All the good states do, and the others can go suck an egg. 

Eggs last a very long time (a few months at least) properly refrigerated and are safe to eat long after the sell date. The egg will lose water, and the air bubble in the end of the egg will continue to grow, so if you put the egg in water and it floats, it's pretty old, but can generally still be eaten if the eggs is not crazy old. 

The membranes will also thin over time, making the yolks easier to break if you're trying to separate them or use them for sunny-side up eggs, e.g. The whites will also thin and the thicker part of the whites won't be as pronounced from the thinner parts. 

Some recipes actually call for aged eggs. Also note that in the US eggs are washed and the outer membrane is removed, so they must be refrigerated. In some other countries the outer membrane is left on and the eggs can be stored safely outside of the refrigerator.