Many, indeed most, worlds have their own local dating system and many have more than one. However, the standard Imperial calendar is in use on most worlds. Within the Imperium it is generally the main dating system, and even outside it is often used as a handy frame of reference.
The system uses a seven-day week (named Oneday, Twoday and so forth) and a 365-day year. Dates within the year are given as Day-Year. For example, 054-1105 is the 54th day of the 1105th year since the founding of the Third Imperium.