I bought a multi-day ticket at the Universal store on May 19th for a May 21st visit. Before purchasing, I asked "Is this good for early entry?" because if it wasn't, I would have just gotten the 1-day. The cashier said it was and that the 1-day was not. When I got to the park, I was told it wasn't good for early entry. After telling them that the Universal store said it was and showing my receipt to prove I bought it there, they let me in anyway.
If you want early entry, don't buy your tickets from the store at the airport!