Javascript beginner here: how do I implement time such as hours and days?

So I'm making an event driven game (at least I think that's what they call the contrary of a "real-time" game), and I want to make days pass, along with the hours on each day. Suppose I click on a button (i.e.: sleep) and time passes by 8 hours, making the hours go forward by 8, and passing by the midnight, the day also changes. I made a simple code that permits the hours go by, and the days go by each 24 hours. The problem is that if I add more hours, the timer resets to 0, so I lose the extra hours. So, using the "sleep button" example, let's say it's currently 22h, and I click the sleep button which should advance the game by 8 hours. The problem is that when it passes midnight, the timer resets to 0. How can I add those lost 6h? Here's the code I wrote:

 var currentHour : int = 0;
 var currentDay : int = 1;
 var currentMonth : int = 1;
 var hoursPerDay : int = 24;
 var daysPerMonth : int = 10;
 var monthCheck : boolean;
 
 function Update () {
 
 if (currentHour >= hoursPerDay)
     {
     currentHour = 0;
     currentDay += 1;
     }
 }