In the bible it tells us in the third chapter of Luke, that Jesus was baptized in the 15th year of the reign of Tiberius Ceasar.
Historically, researching through official Roman documents, the 15th year of Tiberius Ceasar was in 27 AD.
The bible also tells us that King Herod the Great of Judea tried to kill Jesus at his birth, but failed to do so, then later died.
Historically, King Herod the Great of Judea, the one who tried to kill the infant Jesus, died in 4 BC.
So using this historically documented information, we can ascertain that Jesus was born prior to the death of Herod in 4 BC, and we know that Jesus died on the cross 3.5 years after his baptism in 27 AD, so that would be either 30 AD or 31 AD.
And since we know that Jesus was baptized at age 30 and then died at age 33, the life span of Christ is easy to figure out.