High Noon is obtained on the map "Sanctum of Shadows", a map in Chapter 2. I played the map in custom mode, but it should work either way. Note: The achievement's time goes by your computer's actual clock.

Once on the map, continue through it until you reach an outside area. This area has two structures one might consider a "clock tower", but the one you need is the smaller one with a bell in it. It appears in the image below.


When I obtained the achievement, I stood inside the structure next to the bell itself. As soon as my clock hit 12:00 noon, the following text appeared at the top of the screen, and XP shot out from the bell. The achievement implies that it can be obtained at midnight as well.