static/DropboxWorker.js

// intervalBegin()
function intervalBegin(){
  console.log('cheeckingg')
    // minutes= min *60*1000
    // intervalAPI= setInterval( function() {
    
            // const delay = ms => new Promise(res => setTimeout(res, ms));
            ajax("/refreshDropbox")
            .then(function(data) {
                self.postMessage(data); // Code depending on result  

            })
            
    // },3600000) // 60*60*1000 
}

function ajax(url) {
  return new Promise(function(resolve, reject) {
    var xhr = new XMLHttpRequest();
    xhr.onload = function() {
      resolve(this.responseText);
    };
    xhr.onerror = reject;
    xhr.open('POST', url);
    xhr.send();
  });
}

this.now = new Date();
this.hours = this.now.getHours();
this.minutes = this.now.getMinutes();

var millisTill10 = new Date(this.now.getFullYear(), this.now.getMonth(), this.now.getDate(), 9, 0, 0, 0) - this.now;
console.log(this.now)
if (millisTill10 < 0) {
     millisTill10 += 43200000; // try every 12 hours
}
setTimeout(function(){intervalBegin()}, millisTill10);