I've taken a guess at the schedule before and this falls right in line with what I thought.
June 28-30: Duty Finder stress test (but we know now that this was pushed back even though we're still getting a test this weekend.
July 5-14: Week-long extended play stress test. My guess is this would conclude Phase 3. Hence the mass of invites being given out today.
July 19 - August 11: A three week open beta to once again stress the servers for extended play and iron out any last-minute tests.
August 20: Early access begins. They have over a week to distribute the release client to Legacy users and ready themselves for relaunch.
To me this kind of schedule makes the most sense. Not official, just a hunch. So I do believe the 14th lines up with an ideal time to end Phase 3.
On the Beta Forums the schedule says that Duty Finder stress test will not be conducted during the third test, and it will be reschedule for the fourth test
. That being said, my guess is that this will certainly delay Beta phase 3 and thus making the Beta phase 4 two week long instead of three.