The Gregorian calendar does attempt to count the number of years since Jesus.
The Bahá'í calendar counts since Báb had his first religious experience [0]
The Bengali calendar counts (possibly) since the reign of King of Gour, Shashanka [1]
The Buddhist calendar counts since Buddha attained parinibbāna [2]
The Hindu calendar Kali Yuga counts since Krishna left Earth to return to his abode [3]
I'm sure there are more, I only looked at a few. Look how many calendars there are that are in use: http://en.wikipedia.org/wiki/List_of_calendars
Also, I don't think explaining anything can make a belief system collapse.
[0] http://en.wikipedia.org/wiki/Bah%C3%A1%27%C3%AD_timeline [1] http://en.wikipedia.org/wiki/Bengali_calendar [2] http://en.wikipedia.org/wiki/Buddhist_calendar [3] http://en.wikipedia.org/wiki/Kali_Yuga