TimeInterval extension with literal intervals commonly needed

TimeIntervalConsts.swift

extension TimeInterval {
    static var second: TimeInterval { return 1 }
    static var minute: TimeInterval { return 60 }
    static var hour: TimeInterval { return 60 * self.minute }
    static var day: TimeInterval { return 24 * self.hour }
    static var week: TimeInterval { return 7 * self.day }
}