inside of
Within a given range of distance or period of time. We were inside of a mile from town when we ran out of gas. We should arrive at grandma's house inside of an hour.
inside of
Within, in less than the whole of, referring especially to a period of time. For example, They promised to return it inside of a month. Although some authorities believe inside alone conveys the same meaning, the full term is widely used. [Early 1800s]