Though we count with the ordinals (first, second, third), we mark time with the cardinals (0, 1, 2). All of us have assimilated this way of thinking, whether we appreciate it or not. After a baby finishes his 12th month, we all say that the child is one year old; he has finished his first 12 months of life. If the baby turns one when she’s already lived a year, isn’t the only consistent choice to say that the baby is zero years old before that time? Of course, we say that the child is six weeks old or nine months old instead—a clever way of getting around the fact that the baby is zero.
