Two of the most accurate online clocks that keep real time are available at Time.gov and Time.is. Both of these clocks are synchronized to Greenwich Mean Time for absolute precision.
Both of these clocks are available on a number of platforms, ensuring that the need for plug-ins or specific browsers is unnecessary. Synced to an atomic clock and two time agencies in the United States, these clocks are generally precise to between 0.02 and 0.10 seconds, depending on Internet speed and the capacity of the computer or device. Each clock also offers the ability to check and compare the time in different locations and time zones, automatically adjusting for daylight saving time.