Every day, when the game time reaches 8 PM, the rings of the sundial start to spin one by one as the sun shines through the hole in the crystal. Falling sparkles from the sky also make it look like it's snowing both around the sundial and in the town itself. The sparkles stop after ten minutes, and the rings themselves stop spinning one by one just after 9 PM.
In the post-game, after the player defeats Serena or Calem at Kiloude City, the player learns that Professor Sycamore is waiting at the sundial with information on the Mega Ring (he will not appear until the rival is defeated). At the sundial, Professor Sycamore tells the player that he has discovered a way to level up the Mega Ring. He tells the player to touch the sundial with the Mega Ring; after doing so, he reveals that the Mega Ring will now resonate with Mega Stones that are underground. He explains that you can only find those Mega Stones while the sundial is active between 8 and 9 PM; it will not work at any other time.
Sincle Anister City Sundial is only active between 8:00-9:00 PM, and you need to touch the sundial with mega ring to upgrade it. So mega ring will be able to detect mega stones.
I think the Mega Ring receives power from Anistar Sundial and Anistar City Sundial is only between 8-9 O'Clock. So Mega Ring conly recieves power if the sundial can you can only find mega stones between 8-9 O'Clock.
When the Sun hits Anistar City Sundial, It starts working.
At first, Kalos and Hoenn isn't the same region, Maybe Steven Upgraded it, then he give it to the player. Since there are no Sundial in Oras, I think Steven upgraded it by an unknown way so you can find with bigger time spectrum.
The game and the anime isn't the same thing, anime is illogical, they just wants that children will love Pokemon, they will buy it anfd Game Freak and the Pokemon company will earn money. The main character Ash Ketchum, doesn't have neither a Mega Ring nor a pokmon that can Mega evolve. Remo and Alain both went to search a Mega stone, but they didn't use Mega Ring to find. Maybe because they already knew the location of Ampharosite.
Lol, actually, Steven gives you a Mega Bracelet, not a Mega Ring. This means you don't need to upgrade to find mega stones.
Mega Bracelet can still find mega stones without upgrading.
They are only avaliable on a certain time because Game Freak wants to make the game more enjoying, not boring. If you can get all mega stones in 10 minutes you will be happy, but you won't wait the full day for getting mega stones. Game Freak wants people will play their game too much.
If you feel this doesn't fully answer this question, just tell me to hide it. I'm dead.
I only want to say one thing, Game Freak logic, Game Frea king logic