Pomodoro Clock not Stopping after countdown

The clock works until it gets to zero which is where it is supposed to start the break timer. the issue is that it gets to zero then starts counting down in negative numbers. here is the function that has that code. By the way, I have the interval at 100 so I do not have to wait a long time when testing the clock.

//This function is the culprit
function start(){
   $("#start").addClass("disabled");
   $("#myreset").addClass("disabled");
   var secs = Number("59");

   var min = document.getElementById("sessiontime").innerHTML; 
   min = min-=1;

   if(min > -1){
      startcounter = setInterval(function(){
      secs--;

         if(secs > 9){
            document.getElementById("mytimer").innerHTML = min +":"+ secs;
               } else if(secs >=0 && secs < 10){
                  secs = "0"+secs;
                  document.getElementById("mytimer").innerHTML = min +":"+ secs;
                     } else if(secs < 0){
                        min--;
                        secs = 59;
                           } else if(secs === 0 && min === 0){
                              clearInterval(startcounter); 
                              var x =      document.getElementById("arrownumid").innerHTML;
                        mybreak(x);
                     }
      },100);

   }
}