title: convert firestore timestamp to date into different format
body: <p>the timestamp variable is stored in the firestore , and when extracted it is then in the format of seconds and nanoseconds , how to convert the timestamp to the format like 2018-09-19T00:00:00 </p>
  
  <pre><code>let Ref=firebase.firestore().collection("Recruiter").doc(u.uid).collection("Jobs")
      Ref.orderBy("timestamp", "desc").onSnapshot(function(snapshot){
        $.each(snapshot.docChanges(), function(){
          var change= this
          if(change.type==="added"){
             var ab= new Date(change.doc.data().timestamp) 
             console.log(ab)
            thisIns.RecruiterChart.chartOptions.xaxis.categories.push(
              ab
              ) 
  
             console.log( thisIns.RecruiterChart.chartOptions.xaxis.categories)
          }
  
        })
      })
  </code></pre>
  
  <p>the variable ab is showing "invalid date" on the console</p>