I'd have to say this is where Karma needs a good swift smack upside the head. However, you also need to say something to him immediately. "Look, we schedule this time for US, that you choose this time to chat online, text, etc, is incredibly rude, disrespectful and hurtful!" He is fully capable of saying, "sorry, I'll talk to you tomorrow, it's date night with Mo."
Now, I can see where, he's waiting for you so he jumps on the computer and then gets wrapped up in whatever. This is where you tell him you are now available and you'll give him 5-10 minutes(per-determined time limit) to wrap up whatever he's doing. My husband can get so wrapped up in something that he won't even notice that I'm standing at his elbow waiting for him to finish something. I have to be extremely blunt and to the point (as few words as possible).