The rumors are that the iPhone 6 will be released September 20 of 2013, with a formal announcement to confirm this on September 6 2013. The confusion is that the iPhone 5S might be released first and it is also rumored to be released around the September timeline. If the iPhone 5S is released earlier than this, then the iPhone 6 should keep the September dates, although it might be later in the year or early in 2014.
Resource: Know Your Mobile