The Network Time Protocol (NTP) is used to synchronize the time of a computer client or server to another server or reference time source, such as a radio or satellite receiver or modem. It provides client accuracies typically within a millisecond on LANs and up to a few tens of milliseconds on WANs relative to a primary server synchronized to Coordinated Universal Time (UTC) via a Global Positioning Service (GPS) receiver, for example.
WMTime is a Window Maker dock app that displays the time and date and gives you some nice additional features too. It provides a realtime morphing interface (analog <> digital mode), auto-scaled and anti-aliased hands, localization that displays the day and date in various languages, and three user-defined launch commands.
Gameclock is a fairly simple application designed to track the time spent thinking by the players during a chess game. Various clock engines are available (speed chess, fisher chess, board games, or hourglass). The graphical interface is keyboard-driven and is intended to be minimal and simple. The code is made to be extensible to other game types.
Screwdriver++ is a package with various tools for the C++ programmer in Unix environments. It contains classes for time measurement, image and file manipulation, and arbitrary precision arithmetics. Timer is a class for measuring time intervals with millisecond accuracy. Image is a class that uses OpenCv to manipulate images. File is a class to manipilate file input and output in various formats. Number is a class to manipulate arbitrary precision arithmetics.