When determining if one can claim a child as a dependent, what is meant by "more than half the year"? Does it go by calendar months or actual number of days?

If a child leaves home a few days before the end of June without telling anyone where they are, leaves all the stuff behind for a time,  but child is still on parents car insurance, health insurance, mail still coming to home,  etc,  and then at some point in July decides they are not returning what is considered the day they actually stopped being supported?