You're looking at an outdated CBA provision, those were players drafted before 2013. The date is August 15th for one, this comes up every year with the unsigned guys that become free agents, so it's not really up for dispute if the date is June 15th or August 15th.
College Players.
(iii) If a Player drafted at age 18 or 19, who had received a Bona Fide Offer in accordance with Section 8.6(a)(ii) above, becomes a bona fide college student prior to the second June 1 following his selection in the EntryDraft and remains a bona fide college student through the graduation of his college class, his drafting Club shall retain the exclusive rights of negotiation for his services through and including the August 15 following the graduation of his college class.
For purposes of the above provisions, the term "graduation of his college class" shall mean the class with which the Player is scheduled to graduate during his final semester of attendance (as opposed to his matriculating class (the class with which he is expected to graduate as of the date of his original enrollment)). For purposes of clarity, a Player's graduating class may change during his tenure in college.
The only murkiness comes from players that perhaps graduate in 3 years instead of 4 years based on the "For purposes of clarity" that suggest a player may be able to rush their graduation in order to become a Free Agent earlier.