Historians use different calendars based on their religious preferences. Since B.C and A.D stand for Before Christ and Anno Domini (in the year of the Lord) respectively, these abbreviations are most used in religious uses. B.C.E and C.E (Before Common Era and Common Era) are mostly used in education settings as more modern versions of B.C and A.D. The answer to this question would be that some groups make calendars to measure time, while some groups use the calendars in religious instances.
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