How do I create a new date using 2 dates

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Javascript

I have 2 dates and I need to create a 3rd date using those 2 dates.
If

presentdate = "11/30/2018" and enddate = "1/29/2019", then newdate = "12/30/2018"

Since, JavaScript date months range from 0-11 and also the dates are kind of messy, I am not getting the desired results.

What I have tried:

/Step1:
  var sDt = new Date(startDate);
  var pDt = new Date(presentDate);

//Step2: Doing Step 3 directly returns invalid date so using the following step worked in returning dates
  var presentDt = (pDt.getMonth() + 1) + '/' + pDt.getDate() + '/' + pDt.getFullYear();
  var sDt = (sDt.getMonth() + 1) + '/' + sDt.getDate() + '/' + sDt.getFullYear();
 
//Step 3

  var mPresent = moment(presentDt, "MM/DD/YYYY");
  var mStart = moment(sDt, "MM/DD/YYYY");

//Step 4
  // Create a new momnt object compliant with your needs
  var pastStart = moment({
    year: mPresent.year(), // get presentDt's year
    month: mPresent.add(1, 'month').month(), // get presentDt's month
    date: mStart.add(1, 'day').date() // get startdt day of the month and add 1 day to it
  });

  // Display the result in the format you need
  console.log(pastStart.format("MM/DD/YYYY"));

});

This also returns invalid date if my date is "1/30/2019" and endate ="5/30/2019" because when you take end day + 1 and create new Date February doesn't have 31 days. And itr returns Invalid date.

Any help is appreciated. Thank you.