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Why we use B.C. and A.D.

Dear Matthew Alice:

Why do we measure dates from B.C. to A.D.? Where did this system come from? I know it’s measured from the time of Jesus’s birth, but who decided that? And what did people do before then to measure time, or didn’t they care?

— Mike, via email

Of course they cared. We’ve been run ragged by clocks and calendars pretty much forever — at least ever since we started growing crops and living under grand rulers. The A.D.-B.C. system dates from about 525 A.D., invented by a monk who was charged with figuring out a neat way of predicting the date of Easter. Since he was a man of the church, he took as the starting point for his calendar the birth of Jesus, the most significant event in the ecclesiastical year.

This is pretty typical of the many other calendars devised before and since the monk’s. They all begin with a watershed event. The Mayans developed a calendar that went back through thousands of years of mythology, so the cornerstone event wasn’t necessarily an actual occurrence. Egyptians measured years by giving them a number and the name of the pharaoh who was in power at the time. Babylonians used a similar system. The Jewish calendar is measured from a calculation of the Era of Creation. The Muslim calendar begins with the Era of the Hijra, the time Mohammed and his followers migrated from Mecca to Medina. Rome measured time from the founding of the city. So, if you want to develop a Mike calendar — Before Mike-After Mike — go right ahead.

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Dear Matthew Alice:

Why do we measure dates from B.C. to A.D.? Where did this system come from? I know it’s measured from the time of Jesus’s birth, but who decided that? And what did people do before then to measure time, or didn’t they care?

— Mike, via email

Of course they cared. We’ve been run ragged by clocks and calendars pretty much forever — at least ever since we started growing crops and living under grand rulers. The A.D.-B.C. system dates from about 525 A.D., invented by a monk who was charged with figuring out a neat way of predicting the date of Easter. Since he was a man of the church, he took as the starting point for his calendar the birth of Jesus, the most significant event in the ecclesiastical year.

This is pretty typical of the many other calendars devised before and since the monk’s. They all begin with a watershed event. The Mayans developed a calendar that went back through thousands of years of mythology, so the cornerstone event wasn’t necessarily an actual occurrence. Egyptians measured years by giving them a number and the name of the pharaoh who was in power at the time. Babylonians used a similar system. The Jewish calendar is measured from a calculation of the Era of Creation. The Muslim calendar begins with the Era of the Hijra, the time Mohammed and his followers migrated from Mecca to Medina. Rome measured time from the founding of the city. So, if you want to develop a Mike calendar — Before Mike-After Mike — go right ahead.

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4S Ranch Allied Gardens Alpine Baja Balboa Park Bankers Hill Barrio Logan Bay Ho Bay Park Black Mountain Ranch Blossom Valley Bonita Bonsall Borrego Springs Boulevard Campo Cardiff-by-the-Sea Carlsbad Carmel Mountain Carmel Valley Chollas View Chula Vista City College City Heights Clairemont College Area Coronado CSU San Marcos Cuyamaca College Del Cerro Del Mar Descanso Downtown San Diego Eastlake East Village El Cajon Emerald Hills Encanto Encinitas Escondido Fallbrook Fletcher Hills Golden Hill Grant Hill Grantville Grossmont College Guatay Harbor Island Hillcrest Imperial Beach Imperial Valley Jacumba Jamacha-Lomita Jamul Julian Kearny Mesa Kensington La Jolla Lakeside La Mesa Lemon Grove Leucadia Liberty Station Lincoln Acres Lincoln Park Linda Vista Little Italy Logan Heights Mesa College Midway District MiraCosta College Miramar Miramar College Mira Mesa Mission Beach Mission Hills Mission Valley Mountain View Mount Hope Mount Laguna National City Nestor Normal Heights North Park Oak Park Ocean Beach Oceanside Old Town Otay Mesa Pacific Beach Pala Palomar College Palomar Mountain Paradise Hills Pauma Valley Pine Valley Point Loma Point Loma Nazarene Potrero Poway Rainbow Ramona Rancho Bernardo Rancho Penasquitos Rancho San Diego Rancho Santa Fe Rolando San Carlos San Marcos San Onofre Santa Ysabel Santee San Ysidro Scripps Ranch SDSU Serra Mesa Shelltown Shelter Island Sherman Heights Skyline Solana Beach Sorrento Valley Southcrest South Park Southwestern College Spring Valley Stockton Talmadge Temecula Tierrasanta Tijuana UCSD University City University Heights USD Valencia Park Valley Center Vista Warner Springs
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