It is conceptual, not due to the lack of any numeral. The year that start an era is the first year of this era, hence is numbered year 1... While Revolutionary French were perfectly aware of how drawing a "0" which had a great importance on the decimal system they generalized everywhere, they established 1789 as the Year 1 of freedom and 1792 as year 1 of the Republic for those conceptual reasons. And I presume -without knowing it- that when the Jesus Christ's birth era was conceptualised, erudites who conceptualised it did known the 0.
God job we cleared that up we were in danger of thinking Imperator ended in a completely different era due to this inaccuracy