Some hints were left in the WWDC 2014 keynote that Apple will be releasing of the iPhone 6. Based on bits and pieces of information and rumors, iPhone 6 is coming in the market in September.
There at WWDC 2014's keynote, Apple announced iOS 8 - the improved upcoming version of iOS 7. This operating system will be preloaded on the new iPhone. The iOS 8 beta is there and available for developers, it was rumored than general users will get iOS 8 in September. This leads us to conclude that Apple will follow its time frame that it has worked out and is not going to deviate as per its tradition of past two years. Logically the iPhone 6 will not be earlier than iOS8 and Apple isn't going to change the sequence of actions it has planned.
If we go by the announcements by Apple about different actions we are right to draw the deduction that the iPhone 6 release date cannot be before mid September. The iPhone 4s was released on October 14, 2011, and iPhone 5 was released on September 21, 2012, while iPhone 5s has the release date September 20, 2013. Keeping this track of events in view that iOS 8 will be launched in middle of September and iPhone 6 will come after it so it is mid September or slightly later.