Well, that's the absolute deadline to be able to play (part of) that season.
But isn't it a little bit more hairy?
If I understand the CBA then it says that for the prior team to receive right of first refusual or draft compensation they must present the player with a qualifying offer no later than June 25:th (or first monday after the draft).
That offer cannot be accepted earlier than July 1:st.
If no such offer is made, the player becomes a UFA.
The offer terminates automaticly at July 15, but can be extended.
We then enter the potential 'enemy' offer territory, where a new team can present the player with an offer, and the prior team must match it within 7 days.
And finally, if no contract is signed before December 1:st, the player is out for the season
Or am I misreading the CBA?