Seems like an arbitrary starting point. Why not the destruction of the 1st or 2nd Temple? Why not the founding of Israel?Using the bible-a day for a year rule-- Daniel 9:24-27
70 weeks = 490 years
455 bce, the word to restore Jerusalem
406 bce Jerusalem rebuilt
62 weeks=434 years
one week = 7 years
29 ce-Messiah arrives
33 ce Messiah cut off
36 CE end of the 70 weeks.