If you want to mount the clock in a chassis, here's the recommended method. Drill number 4 machine screw holes to line up with the four holes in the corner of the board. Thread 1 inch bolts through from the outside. On the top left bolt, thread a 3/16" spacer. On the other 4 bolts, thread a 5/8" spacer.
Next, separately thread a machine screw through from the top of the Pi through the right hand hole (the one not in a corner). Between the display board and the pi insert a 3/8" spacer. Top the bolt (on the display side of the display board) with a nut.
Add a 3/8" spacer between the pi and the display board and then thread everything over the 4 corner bolts. What you wind up with is a 5/8" spacer on the three corners that don't involve the Pi. On the corner where the Pi is, there's the case back, a 3/16" spacer, then the pi board, then a 3/8" spacer, then the display board then a nut. This works because the Pi board is about 1/16" thick. The remaining bolt doesn't go through the back chassis, it just holds the Pi to the display, with the two separated by a 3/8" spacer.
The 3/16" space between the Pi and the back is just enough for the board and all of the connectors to clear the back, but it's recommended that this mounting method be used only for wood or plastic cases. If you use a metal case, then you probably should add at least another 1/8", making the bottom corner spacer 5/16" and the other corners 3/4". The inter-board spacing (with the recommended GPIO male/female header pair) remains 3/8".
You metric people are on your own. :)