Well, "lord of time" could be given as a title to Xylar, because he does control past present and future unlike any vex, because he truly isn't a vex. He is forged by the darkness rather then born as a vex. The name "timeless" comes from when he banished in time, he doesn't exist in our universe because atheon, along with many other high axis minds in our galaxy put a seal on him, forbidding him from entering our world. So I guess "lord of time" does make sense, but Xylar is well known for being the one who doesn't live in time.
https://www.bungie.net/en/Forum/Post/147510697/0/0
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Good point, he could be really either, just how Crota has differing titles (Son of Oryx, Hive Prince) Xylar could be considered both timeless and the lord of time, controlling time from outside of any known timeline