There was a similar issue for the older cars a year or so ago.
from the thread
I talked to a hyundai dealer today....they told me a satellite broke for certain signals, and if the reset button didnt work, would have to wait till they fixed the satellite...said they are aware of the issue, and it should automatically reset to normal when they fix the satellite. He said it had to do something with the blue link update...he said to be patient...lol
might not be the same thing. but it appears to happen since 2015 judging by the thread here.

Problem with Navigation unit clock settings

My '15 sonata Limited w/ Tech package has been having some strange issues appear with the clock on the Navigation unit. A few weeks back, I noticed that the Date/time display across the top of the navigation screen was showing May 28. When I dug down into the clock settings, it was pulling the...

www.hyundai-forums.com

 

Thanks. I'm going out tonight to pick up burgers. Maybe it will be working again. That's kind of stupid if it needs a satellite to tell time. Oh for the good old days when a clock was just a clock and needed nothing but the battery.

 

The clock is back. Weird.

 

I have a 21 Hyundai Kona. Went out day and there was no clock, only dashes. Saw where a girl fixed hers by inserting pin in small hole by pwr/vol control knob. Worked like a charm!