There are myriad questions about high-speed trading, including whether such traders have unfair advantages in the stock market. Here, The Wall Street Journal explains a problem that dramatically illustrates how fast it really is. "Clock drift" is the common term for timepieces moving slightly out of sync. In another industry, that might barely be noticeable, let alone a problem. In the stock exchange, not being able to pinpoint changes down to the millisecond can make it hard to track data or even provide cover for illegal trading. And regulators are still figuring out how to fix the problem.