The stardate is a comprehensive futuristic calendar system, used in the Federation that allows planets and outposts that are light years apart and starships traveling at near-relativistic velocities to keep track of a unified time base despite local and unforeseen changes in the speed at which time passes. The stardate system was introduced in the Federation sometime between 2161 and 2265.
This time system adjusts for shifts in relative time which occur due to the vessel's speed and space warp capability. It has little relationship to Earth's time as we know it. One hour aboard a starship at different warp speeds may equal many Earth hours. Stardates specified, in a log entry for example, must be computed against the speed of the vessel, the space warp factor, and its position within our galaxy in order to give a meaningful reading when read back.
Stardate units standardize time measurement between various cultures, worlds and races. Stardates are calculated through a complicated equation that takes into account relativistic effects, universal expansion and the effects of gravity on time and space. Every so often the stardate system has to be updated to take into account the expansion of space or other natural effects. Starfleet vessels did this by contacting a nearby Starfleet time beacon and adjusting their internal chronometer. (TNG: "Cause and Effect")
Stardates were in use as far back as the 2150s, although Earth had not yet adopted the system. Among the species who did utilize stardates at the time were the Xindi. In 2154, Degra, a Xindi-Primate, sent a coded message to Enterprise containing a stardate for when Enterprise should rendesvous with Degra's ship at specified coordinates, which were also contained within the message. Vulcans also seemed to have an understanding for stardates at that time, as T'Pol was able to calculate that the given stardate was three days away. (ENT: "Damage")
In the 24th century, approximately one thousand (1,000) stardate units pass for every Earth year, although there are variations in the system at some points in the timeline.
In an alternate timeline, the combat date replaced the stardate as the dating system used by Starfleet during their war with the Klingon Empire. This was indicative of the militaristic nature of Starfleet in this timeline.(TNG: "Yesterday's Enterprise")