Stardates were a common used time keeping dates utilised by the people of the United Federation of Planets.
At least four digits long the last three including any decimals indicate a date within a given year. The other digits indicate the year.
For example, 52345.45, "52" refers to the year (2375) and the rest of the numbers indicates roughly a third of the way through the year.
The exact nature of stardates have never been explained on screen and as such they are used somewhat haphazardly. While it is generally accepted that 000 is the start of the year and 999 is the end of the year (though there is some merit to that this is not always the case) the numbers inbetween do not seem to be evenly distributed, such that the time gap between XX001 to XX002 might not be the same as XX101 to XX102.
For example, three months are said to have passed between the end of the fifth season of DS9 and the first episode of the sixth. The stardate 50975.2 is referenced in "Call to Arms" and that occurs a day or two before the end of the episode yet in the second episode of season six. "Rocks and Shoals" the stardate 51096.2 is given. If they were evenly distributed than "Call to Arms" occurs around the 22nd of December and "Rocks and Shoals" on the 5th of February, a gap of roughly 6 weeks, half the time that is supposed to have passed. This suggests that the gap between individual stardates might not be uniform.
Star Trek: The Farthest Star uses an evenly distributed measure for stardates for simplicity's sake. This does provide some conflict with the DS9 stardates and the series attempts to remain consistent with them but in some cases they might be changed for internal consistency. The series also keeps to 000 being the start of the year and 999 the end.