Need to add a variable number of months to a start date and calculate an accurate end date

So I have a number of months and 3 date fields (for both start and end) - I have a function in progress that kinda works (from a developer/js hack, not me) which is supposed to add the number of months (term) to the start date to set the 'end date' to the correct date... but its using math to create dates instead of a true calendar so if I put in 12/31/2000 with a term of 12 months (include the start month as one so +11 mos) it returns 11/31/2001 - which doesn't exist (Nov. has only 30 days) - so I'm open to a complete rewrite - but it needs to behave properly and return a real date in the 'end date' - requirements are to run the calculation on change of any of the four fields (term, start_date_month, start_date_day, start_date_year) - if any of these four fields are empty the function should not run, if a non number is entered an alert should pop up saying "enter a valid number" and when we finally run our date addition - we should return a real date and pop it into the 3 end date fields.

Heres the current code - again - i'm open to a 'fix' or a rewrite so long as it works and covers at least what the following does:


<script type="text/javascript">
      
function calcTermDate () {
      
      var numOfMonths = document.getElementById('schednoteTerm').value;       
      var month = document.getElementById('schednoteTermCommenceDateMonth').value;
      var day            = document.getElementById('schednoteTermCommenceDateDay').value;
      var year       = document.getElementById('schednoteTermCommenceDateYear').value;
      
      if (year >= 1999 && year <= 2025) {      

       if ((month && month <= 12) && (day && day <= 31) && year && numOfMonths) {                  

            //Joining three date fields to create a commencement date
            var newDate = new Date(month + "/" + day + "/" + year);
            var d                   = new Date(newDate);                               
            var day             = d.getDate();
            
            //Adding term to commencement date
            d.setMonth(d.getMonth() + parseInt(numOfMonths));             
            if (d.getDate() < day) {
              d.setDate(1);
              d.setDate(d.getDate() - 1); //End of Term date
            }            
            
            // Spliting into three date values
            var newDay                   = d.getDate();
            var newMonth             = d.getMonth();
            var newYear             = d.getFullYear();      
       }
      }
            
      // Error check
      if (isNaN(month) || isNaN(day) || isNaN(year)) {
            alert ("Invalid Date");
      }                   
      
      // Populating End of Term date values if created
      if (newDay && newMonth && newYear) {      
            if (document.getElementById('schednoteTermCommenceDateMonth')) {
                  if (newMonth < 10) { newMonth = '0'+newMonth}
                  document.getElementById('schednoteTermEndDateMonth').value = newMonth;
                  document.getElementById('endMonth').value = newMonth;
            }
            if (document.getElementById('schednoteTermCommenceDateDay')) {
                  if (newDay < 10) { newDay = '0'+newDay}
                  document.getElementById('schednoteTermEndDateDay').value = newDay;
                  document.getElementById('endDay').value = newDay;
            }
            if (document.getElementById('schednoteTermCommenceDateYear')) {
                  if (newYear < 10) { newYear = '0'+newYear}
                  document.getElementById('schednoteTermEndDateYear').value = newYear;
                  document.getElementById('endYear').value = newYear;
            }
      } else {
                  document.getElementById('schednoteTermEndDateMonth').value = '';
                  document.getElementById('endMonth').value = '';
                  document.getElementById('schednoteTermEndDateDay').value        = '';
                  document.getElementById('endDay').value        = '';
                  document.getElementById('schednoteTermEndDateYear').value       = '';
                  document.getElementById('endYear').value       = '';
      }
}// End of calcTermDate
</script>


                     <div>Term: (Months)<br><input  type="text" id="schednoteTerm" name="schednoteTerm" value="" onChange="" onBlur="calcTermDate();"></div>
                        <div>Start Date:<br>
                              <input  type="text" id="schednoteTermCommenceDateMonth" name="schednoteTermCommenceDateMonth" maxlength="2" value="" onChange="calcTermDate();"> /
                              <input type="text" id="schednoteTermCommenceDateDay" name="schednoteTermCommenceDateDay" maxlength="2" value="" onChange="calcTermDate();"> /
                              <input type="text" id="schednoteTermCommenceDateYear" name="schednoteTermCommenceDateYear" maxlength="4" value="" onChange="calcTermDate();">
                        </div>
                        <div>End Date:<br>
                              <input type="text" id="schednoteTermEndDateMonth" name="schednoteTermEndDateMonth" maxlength="2" value=""> /
                              <input type="text" id="schednoteTermEndDateDay" name="schednoteTermEndDateDay" maxlength="2" value=""> /
                              <input type="text" id="schednoteTermEndDateYear" name="schednoteTermEndDateYear" maxlength="4" value="">
                        </div>
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