How I can move the clock, by default at the far right side of the menu bar, to the middle of the menu bar?
Unfortunately for you, the answer is: you can't. The way the Mac OS X Menu Bar is set up, Application Menus get added starting in the left hand corner (after the Apple menu) and going right; System and 3rd party Menu Extras get added from the right hand corner and go left. There's no way to absolutely position a menu on the menu bar; you can only change the order of the items.
As Tony mentioned in his answer, the user can reorder Menu Extras by dragging them while holding Command. Only the developer of an app can reorder the Application's menus. But there's no way to position anything in the center.