Lots of films have staggered release dates, and there are lots of reasons for this, whether it's fitting in with seasons or other releases, country of origin, expected interest, and so on.
However, Star Wars: The Force Awakens is a US film (in so much as any film is), it's likely to be in the top three grossing films of all time (up to now), and the modern world is full of spoilers that pay no attention to country of origin (e.g. no Twitter during Empire's famous reveal).
Why is the US release datetime over a day later than so many other locations?
(It was released in a lot of countries on the 16th, most countries by the 17th, and the US and Canada on the 18th. Hardly anyone (China, Pakistan) get it later than this).