Sounds like you're using a font with proportional number characters. For fixed-width fonts, each character uses the same number of pixels (in width), but most fonts are proportional, and each character is only as wide as is necessary. But many proportional fonts use fixed-width numbers. The Arial font appears to be one of them. Therefore, the width of the clock remains the same no matter what time is on the clock (with the exception of, for example, going from 10:00 to 9:00).
To work around this, you can set the width of the cell that holds the clock. You'll want to set it wide enough that it doesn't cause the cells around the clock cell to resize with each clock tick.
But you'll still probably have another issue: because of the proportional numbers, the width of the clock itself will change with each tick of the clock. Since the clock is centered, it will move to re-center itself. You'll want to change the alignment from "center", probably to "left".
Best solution is to use a different font, but that's up to you.