Mechanical clocks, such as a wristwatch, rely on springs and mechanisms to keep track of time; digital clocks use crystals and radio signals to do the same. Although mechanical clocks frequently run fast or slow, it's not common for a digital clock to run fast or slow. If a digital clock keeps track of time inaccurately, there are several issues that could be at fault.
Digital clocks use an oscillator to track time. Typically, a digital clock's oscillator comes in the form of a glass crystal that rotates and slightly changes shape when an electric charge passes through. When the crystal changes shape, it emits a faint sound that is transformed into an electronic signal on a fixed frequency and 60 Hz. A series of adjustable counters reduce the crystal's oscillation to a rate of 1 Hz per second, which his fitting because 1 Hz is representative of one rotation per second. At the rate of 60 rotations per minute, the digital clock is set to keep track of time. If adjustable counters are set to the slightest variance in either direction, the clock will run fast or slow.
Atomic clocks are digital clocks that rely on radio signals to ensure accurate time keeping. These types of clocks operate in the same manner as a standard digital clock, but make radio contact with the National Institute of Standards and Technology's atomic clock radio station in Boulder, Colo. This radio station, WWVB, broadcasts on the 60 KHz radio frequency through a 50,000-watt antenna. If an atomic clock runs fast by a minute or two, its crystal oscillator is likely to blame, but if it runs fast by hours, the NIST's broadcast is likely at fault. Depending on how often the atomic clock is programmed to tap into the NIST broadcast, the problem is usually fixed by the next time the clock updates.
Many digital clocks are built into bedside radio units that feature an alarm clock. If you use the device as a method to wake up in the morning, your semi-conscious state of mind can be the cause of your digital clock running fast. For instance, if, in your grogginess, you reach for the "Snooze" button when the alarm goes off and have to press the button twice to get it to work properly, you might be hitting the "Minute" button on the first try instead of the "Snooze" button. It might go unnoticed at first, but in a week or so, it will be evident the clock is running fast.
Although digital clocks run fast or slow less frequently than their mechanical counterparts, it's not uncommon for it to happen. Like a mechanical clock, the only way you can fix a digital clock's inaccurate timekeeping is to synchronize and manually adjust the clock periodically -- unless it's an atomic clock, in which case the radio broadcast eventually will fix the time automatically.