I want to reduce the amount of this code!

//setInterval (String, mmS) 1000mms = 1s
setInterval (clock, 1000); // Reflect results every second

function clock() {
  var weeks = new Array("Sun", "Mon", "Thu", "Wed", "Thr", "Fri", "Sat");
  // Output the day of the week
  var now = new Date();
  variable=now.getFullYear();
  varmo = now.getMonth()+1;
  vard = now.getDate();
  varw=weeks [now.getDay()];
  varh = now.getHours();
  varmi = now.getMinutes();
  vars = now.getSeconds();

  if(mo<10){
    mo = "0" + mo
  };
  if(d<10){
    d = "0" + d;
  }
  if(mi<10){
    mi = "0" + mi;
  }
  if(s<10){
    s = "0" + s;
  }

  vardate=y+"/"+mo+"/"+d+"("+w+")";
  var time=h+":"+mi+":"+s;

  document.getElementById("clock_date").innerHTML=y+"/"+mo+"/"+d+"("+w+")";
  document.getElementById("clock_time").innerHTML=h+":"+mi+":"+s;
  // document.getElementById("clock_date").innerHTML=date;
  // document.getElementById("clock_time").innerHTML=time;
  document.getElementById("clock_frame").style.fontSize=window.innerWidth/10+"px";

  // varget_time = document.getElementById('clock_time'); // Get HTML element object
  // console.log(get_time); // Log out all [object HTMLParagramElement] elements?
  varget_date = document.getElementById('clock_date').innerHTML;
  console.log(get_date); // Output for days only

  varget_time = document.getElementById('clock_time').innerHTML;
  console.log(get_time); // Time only output

  console.log(w);

  if(w==="Sun"||w==="Sat"){//if not Saturday or Sunday
    console.log("Holiday");

    // keep one's chimes
  } else{
    console.log("Weekdays";

    switch(time){
    case "09:30:00" :// 0 limit START
      document.getElementById("Sound").play();
      alert("The 0th period has started.");
      break;

    case "09:45:00" :// 0 limit END · 1 limit START
      document.getElementById("Sound").play();
      alert("The first period has started.");
      break;

    case "10:35:00" :// 1st limit END · Break time START
      document.getElementById("Sound").play();
      alert("The first period is over.");
      break;

    case "10:45:00" :// Break time END · 2nd limit START
      document.getElementById("Sound").play();
      alert("The second period has started.");
      break;

    case "11:35:00" :// 2nd limit END · Break time START
      document.getElementById("Sound").play();
      alert("The second period is over.");
      break;

    case "11:45:00" :// Break time END · 3rd limit START
      document.getElementById("Sound").play();
      alert("The third period has begun.");
      break;

    case "12:35:00" :// 3rd limit END ·Lunch break START
      document.getElementById("Sound").play();
      alert("The third period is over.");
      break;

    case "13:15:00" :// Lunch break END ·Limited 4 START
      document.getElementById("Sound").play();
      alert("The fourth period has begun.");
      break;

    case "14:05:00" :// 4th limit END · Break time START
      document.getElementById("Sound").play();
      alert("The fourth period is over.");
      break;

    case "14:15:00" :// Break time END ·5 limit START
      document.getElementById("Sound").play();
      alert("The 5th period has started.");
      break;

    case "15:05:00" :// 5 limit END · Break time START
      document.getElementById("Sound").play();
      alert("The 5th period is over.");
      break;

    case "15:15:00" :// Break time END ·6 limit START
      document.getElementById("Sound").play();
      alert("The 6th period has started.");
      break;

    Case "16:05:00":// 6th limit END, closing ceremony, etc.
      document.getElementById("Sound").play();
      alert("The 6th period is over.");
      break;

    case "16:15:00" :// Break time END ·7 limit START
      document.getElementById("Sound").play();
      alert("The 7th period has started.");
      break;

    case "17:05:00" ://7 limit END
      document.getElementById("Sound").play();
      alert("The 7th period is over.");
      break;

    case "17:50:00" :// Encourage to leave school    
      document.getElementById("Sound").play();
      alert("It's 10 minutes before I leave school.");
      break;

    case "18:00:00" :// Completely leaving school
      document.getElementById("Sound").play();
      alert("It's time to go home.");
      // Play music
      break;

    default:
      break;

    }
  }
}