The power to instantly communicate with anyone I want, regardless of time or place.
Making people think they're going insane or prank-calling Charlemagne aside, even just the ability to call your future self for advice or warn your past self is stupidly overpowered. In a Many Worlds timeline it can be absolutely insane, Qualia the Purple-style; by constantly reporting back to your past self and telling them about your life, your past self can simply change the decisions you made and live a different life. Then they report back to their past self, and they can change the decisions and live a different life, and so on. You're not even limited to the time after you acquired the power, as you can simply call any version of your younger self even prior to it, although who knows what shenanigans that could cause. By doing so you can effectively tailor and live your perfect life.
Even in a fixed timeline scenario it's handy, since it acts as reality warping (as the conversation with your future self is now destined to happen regardless of your actions). Just call your future self at an age close to when you would like to die (say 150); if you get no answer, count down day by day until you get a reply. Just say hello to each other and you're set; regardless of whatever actions you take, no matter how careless or dangerous they might be, you're guaranteed to live to that point. In short, you have absolute "freedom" because you have absolute certainty. Once you've done that you can test whether you're in a Many Worlds timeline or not to your leisure by calling different versions of yourself.
Even if time works in a different way we haven't even conceived yet, it's still an extremely useful tool that can test and discover the limits of it and our Universe. Not to mention you still have all the perks of telepathy at your disposal for fun.
While playing in God's domain is asking for trouble, if you're gonna get a superpower you might as well see its limits. A power that simply allows myself to help myself, past or future, also really speaks to me.
tl;dr Even minor time powers like this are crazy.