코드 : 

function time_convert(num)
 { 
  var hours = Math.floor(num / 60);  
  var minutes = num % 60;
  return hours + ":" + minutes;         
}

console.log(time_convert(71));
console.log(time_convert(450));
console.log(time_convert(1441));

결과 :

1:11

7:30

24:1