Question

from where BC starts

Answer 1

The 1st century BC, also known as the last century BC, started on the first day of 100BC and ended on the last day of 1 BC. The AD/BC notation does not use a year zero; however, astronomical year numbering does use a zero, as well as a minus sign, so "2 BC" is equal to "year -1".

Answer 2

from Christ's birth I.e the starting of BC