You must have the Dawn & Dusk ability which you can get early in the game.
Now you can just use Dawn & Dusk 30 times in a row to get the achievement quickly. Each time, just hold
and you'll see the day turn to night, or night turn to day (if you're indoors you might only see Bayek sit down and then stand up). You can release
as soon as the animation starts. Then hold it again when you get control back.