Going a step further, you can "encase" your clock in an HTML <div> element, and by giving it the right style attributes you can prevent it from shifting anything around. You'll need to determine the height and width of your clock, but you can experiment to obtain that. Change the HTML of your clock cell to:
<div style="height: 200px; width: 500px; overflow: hidden"><clock></div>
Setting the overflow attribute to hidden tells the div that even if the contents (the clock in this case) is bigger than 500x200 pixels, to crop it (just chop it off). Basically, this means the div will be 500x200 pixels no matter what. You can adjust this until it fits your clock exactly.