Here is the requirement:

I have a Visualforce page with inputfield which is a rich text field. I want to insert a timestamp value where ever the cursor is present. I have a code that runs perfectly on a html textarea [jsFiddle: https://jsfiddle.net/et9borp4/] but I am not able to make it work on Salesforce's inputField.

Here is the code:

<script>
   $j = jQuery.noConflict();
   $j(document).ready(function(){
        $j('.insert-timestamp-btn').click(function(){
            var tstamp = new Date();
            insertAtCursor($j('#notes'),formatDate(tstamp));
        });
   });

      function insertAtCursor(myField, myValue) {
            //IE support
            if (document.selection) {
                myField.focus();
                sel = document.selection.createRange();
                sel.text = myValue;
            }
            // Microsoft Edge
            else if(window.navigator.userAgent.indexOf("Edge") > -1) {
              var startPos = myField.selectionStart; 
              var endPos = myField.selectionEnd; 
                  
              myField.value = myField.value.substring(0, startPos)+ myValue 
                     + myField.value.substring(endPos, myField.value.length); 
              
              var pos = startPos + myValue.length;
              myField.focus();
              myField.setSelectionRange(pos, pos);
            }
            //MOZILLA and others
            else if (myField.selectionStart || myField.selectionStart == '0') {
                var startPos = myField.selectionStart;
                var endPos = myField.selectionEnd;
                myField.value = myField.value.substring(0, startPos)
                    + myValue
                    + myField.value.substring(endPos, myField.value.length);
            } else {
                myField.value += myValue;
            }
        }
    
        function formatDate(date) {
            var hours = date.getHours();
            var minutes = date.getMinutes();
            var ampm = hours >= 12 ? 'pm' : 'am';
            hours = hours % 12;
            hours = hours ? hours : 12; // the hour '0' should be '12'
            minutes = minutes < 10 ? '0'+minutes : minutes;
            var strTime = hours + ':' + minutes + ' ' + ampm;
            return date.getMonth()+1 + "/" + date.getDate() + "/" + date.getFullYear() + " " + strTime;
        }

</script>

<apex:form id="form">
     //Notes__c is rich text field in Salesforce
    <apex:inputField id="notes" styleClass="notesClass" value="{!cnotes.Notes__c}"/>
    <button type="button" class="insert-timestamp-btn">Insert Timestamp</button>
</apex:form>