- Charted Space Chronology
Technically, an aspect of calendar reform, the use of the year zero provides a central date point for counting years back into the Long Night and forward into the future of the Imperium. The actual advantages of a holiday year are slight: centuries begin in the hundredth numbered year instead of the first numbered year, and by selecting a base year numbered zero (instead of the generally selected year 1), the Imperial dating system became a standard number line extending forwards and backward in time. Years before the founding of the Imperium became simple negative numbers, and the differences between dates before and after the founding of the Imperium could be determined by subtraction.
History & Background (Dossier)
More importantly at the time, the holiday year was used as part of the public relations campaigns which were impressing the authority of the Imperium upon local governments. The minor requirement of changing to the new calendar was used to determine cooperation by those governments. See also Dating Systems, and Date Conversion.