score:1

Accepted answer

Use the chart time to format the tooltip:

// Store a reference to the chart
const stockChart = Highcharts.stockChart('container', {

  // ...

  tooltip: {
    formatter() {
      // Use the time object of the chart to format the date
      return `${stockChart.time.dateFormat('%H:%M:%S', +new Date(this.x))}:  <b>${new Intl.NumberFormat().format(this.y)}</b>` 
     }               
   }

  // ...

})

Demo in JSFiddle


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