Eligibility to extend players under contract until Date X

This is something I am unfamiliar with, but continues to be a recurring talking point that Dorion uses when referring to Karlsson, Stone and (correct me if I am wrong) several other UFA's over the past couple seasons.

Dorion has said they are unable to offer a contract to certain players until date X (in Karlsson's case it was July 1st, 2018, in Stone's case it was January 1st, 2019). Are these dates real?; and if so- if it is not super in-depth CBA laws that can't be explained in a few sentences- can someone give me the coals notes on these rules? I am a little confused as to why you can't table contract extensions to players under contract on your own team and that you have the rights to. This talking point has perplexed me a bit.

More than anything, I am just sad that I can't even take the owner or GM's words at face value without being completely skeptical about their honesty.

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