If you're left- or center-aligning then it will jump to the left once the clock drops to under 10 minutes. But not if you right-align. Of course, if your clock is supposed to be centered on the screen, it will be a little off-center once it gets below 10 minutes, but that might be better than shifting to the left.
I'm not sure how your layout is setup, but if you've got the clock in an absolutely positioned <div>, for example, then I would set the width of the div to the max size it would need to be (as many pixels wide as the clock would be at 30:00, for example), center it on the screen using the top and left styles, and set the text-align to right.