The Destiny Beta dates for Xbox 360 and Xbox One players has been finalized. With the PlayStation 3 and PlayStation 4 beta access for Destiny starting on July 17th, Xbox 360 and Xbox One players will have to wait till July 23rd to gain access to the Destiny beta.
Bungie has confirmed that they will be taking the servers down for maintenance on July 21st and 22nd, so PlayStation players will not have access those days, but the beta will resume for both the PlayStation and Xbox platforms on July 23rd. The beta access will end on all platforms at 11:59 PDT on July 27th.
The only way to guarantee access to the beta for Destiny is to pre-order the title from participating retailers. Destiny is still on schedule to launch in September.