Excel serial number conversion to date in javascript

gistfile1.txt

function ExcelDateToJSDate(serial) {
   var utc_days  = Math.floor(serial - 25569);
   var utc_value = utc_days * 86400;                                        
   var date_info = new Date(utc_value * 1000);

   var fractional_day = serial - Math.floor(serial) + 0.0000001;

   var total_seconds = Math.floor(86400 * fractional_day);

   var seconds = total_seconds % 60;

   total_seconds -= seconds;

   var hours = Math.floor(total_seconds / (60 * 60));
   var minutes = Math.floor(total_seconds / 60) % 60;

   return new Date(date_info.getFullYear(), date_info.getMonth(), date_info.getDate(), hours, minutes, seconds);
}

Why this values returns equals datetimes?

Date in Excel: 2018-03-12 08:01:00
ExcelDateToJSDate(43171.33472)
return
"2018-03-12T08:01:00Z"

Date in Excel: 2018-03-12 08:02:00
ExcelDateToJSDate(43171.33403)
return
"2018-03-12T08:01:00Z"