The Anno Domini (A.D.) dating system was proposed in AD 525 by Dionysius Exiguus who wanted to replace the dating system of the Diocletian era as he felt it carried on the memory of Diocletian, who persecuted and oppressed Christians.  The last year of the Diocletian calendar 247 was immediately followed by AD 532.  Dionysius implied that Jesus’s incarnation had occurred 525 years earlier, without stating an exact year of birth.  The A.D. system wasn’t widely used until AD 800.  Birthdays were probably so confusing during those times.  In just one solar year, you could age 285 years on paper.  This stuff really hurts my brain.  But it’s interesting to know that no body ever cited year 28 as his birthday or anything like that.  I guess.