Calendar is the time scale. There had been so many calendars throughout the history. People always counted time in days, months and years with respect to a major event or incident in the past. That way there was no standard calendar. (even today we have people says that a fellow was born just about an year after the death of Indira Gandhi or something like that). Today we have a standard calendar for which the major event of reference that occurred in the history is the birth of Lord Jesus. And it has remained the major incident in the human history till date. Not sure how long this calendar lasts but there's going to come a new calendar after this. I think the next major event that will change the calendar will be Lord Jesus' second coming when the people will count time for 1000 years under the 1000 years' rule.