Time is one of the fundemental units of physics, defining quantities such as velocities. Numerous races have engaged in the study of space and time, though after thousands of years of study, it is the protoss that possess the greatest understanding and even then, full understanding is limited to the preservers.
Time is not linear. It wraps in on itself, congregating around feelings, moments of history and memories. It then scatters carrying the imprint of these things with it. They attempt to merge, succeed, then scatter again, the process repeating itself into infinity. If the X-factor is any indication, the flow of time can be influenced through psionic means.
While the nature of time is constantly shifting and changing, certain patterns emerge in it. One such pattern is the xel'naga's cycle of reproduction which has shaped the history of the universe. The xel'naga themselves could twist time and space using rip fields. A chrono-rift device may also provide a means of manipulating time.
The Terran Confederacy and Terran Dominion had/have a standardized form of timekeeping, using "SCT" and "GMT" designations respectively. The Dominion keeps track of hours with a 27 hour (or longer) system. The United Earth Directorate appears to use a 24 hour day, seven day week standard.
Terrans sometimes used different terms for time units, especially in the military: