This function adds a number of hours to a 24 hour clock:

    /**
     * @param {Integer} now The current hours
     * @param {Integer} add The number of hours to add
     */
    function addHours(now, add){
        var h = (now + add) % 24;
    
        return h < 0 ? 24 + h : h;
    };

 - `addHours(2, 5) //7`
 - `addHours(23, 5) //4`
 - `addHours(23, -5) //18`
 - `addHours(72, 3) //3`
 - `addHours(0, 0) //0`

The output should always be an integer in the range of 0 - 23.

I can't see any problem with it, but maybe you can. Please let me know if it can be improved or if there are any bugs.