import re


def starting_index_timestamp(line):
    """Return the index where the timestamp starts, in a line. If timestamp does not exist, return None"""
    reg_expr_end_timestamp = re.compile(';[0-9]+$')
    res = reg_expr_end_timestamp.search(line)
    if res is not None:
        return res.span()[0]
    else:
        return None