A digital clock is a type of clock that displays the time digitally, i.e. in cyphers, as opposed to an analog clock, where the time is displayed by hands. Usually, digitally clocks are associated with electronic drives. (Analog clocks are driven mechanically or electronically.)
To represent the time, most digital clocks use a seven-segment LED, VFD, or LCD display for each of four digits. They generally also include other elements to indicate whether the time is AM or PM, whether or not an alarm is set, and so on.
One notorious drawback to digital clocks is the difficulty of setting the time in some designs. Most digital clocks flash 00:00 or 12:00 by default when first powered on and, since the clock is often not a critical function in many electronic devices, people often allow them to display this default.
Moreover, since they run on electricity, most digital clocks must be reset every time they are moved or the power is cut off. This is a particular problem with alarm clocks, since a power outage during the night usually results in the clock failing to trigger the alarm in the morning.
To reduce the problem, they often incorporate a battery backup to maintain the time during power outages. More recently, some devices incorporate a method for automatically setting the time, such as using a broadcast radio time signal from an atomic clock, getting the time from an existing satellite television or computer connection, or by being set at the factory and then maintaining the time from then on with a quartz movement powered by an internal rechargeable battery.