Error: Object doesn't support property or method 'split'

Hi Experts,

This is Question 3 of hopefully 3!

My previous questions were to do with combining and formatting a date with hours and minutes to make a comparison between two datetimes:

https://www.experts-exchange.com/questions/28699609/Formatting-date-and-time-JavaScript.html

I now need to split that result back into hours and minutes for a couple of final calculations:

I need to round down the results to the nearest quarter of an hour
and
I need to display the result as a rounded decimal time i.e. two and a quarter hours would be 2.25
so my final code will display three results:
actual time difference (hh:mm)
rounded time difference (hh:mm)
rounded time difference as a decimal (hh.mm)

My code so far gets the following error:
Error: Object doesn't support property or method 'split'

This part of the code is tested and working many thanks to assistance from experts:

function TimeDuration() {

            var therate = document.getElementById('<%=DDLRate.ClientID%>').value;
            var startdate = document.getElementById('<%=TxtStartDate.ClientID%>').value;
            var enddate = document.getElementById('<%=TxtEndDate.ClientID%>').value;
            var starthours = document.getElementById('<%=DDLStartHrs.ClientID%>').value;
            var startmins = document.getElementById('<%=DDLStartMins.ClientID%>').value;
            var endhours = document.getElementById('<%=DDLStopHrs.ClientID%>').value;
            var endmins = document.getElementById('<%=DDLStopMins.ClientID%>').value;
            var startparts = startdate.split('/');
            var endparts = enddate.split('/');
            var mystartdate = startparts[2] + "-" + startparts[1] + "-" + startparts[0];
            var myenddate = endparts[2] + "-" + endparts[1] + "-" + endparts[0];
            var started = (mystartdate + 'T' + starthours + ':' + startmins + ':00.000Z')
            var ended = (myenddate + 'T' + endhours + ':' + endmins + ':00.000Z');
            var diff = daysBetween(new Date(Date.parse(started)), new Date(Date.parse(ended)));

           
            
            if (startdate != '') {
                document.getElementById('<%=Txttest1.ClientID%>').value = started;
            }

            if (enddate != '') {
                document.getElementById('<%=Txttest2.ClientID%>').value = ended;
            }

            if (startdate != '' && enddate != '') {
                document.getElementById('<%=Txttest3.ClientID%>').value = diff;
            }


 function daysBetween(date1, date2) {
            //Get 1 day in milliseconds
            var one_day = 1000 * 60 * 60 * 24;

            // Convert both dates to milliseconds
            var date1_ms = date1.getTime();
            var date2_ms = date2.getTime();

            // Calculate the difference in milliseconds
            var difference_ms = date2_ms - date1_ms;
            //take out milliseconds
            difference_ms = difference_ms / 1000;
            var seconds = Math.floor(difference_ms % 60);
            difference_ms = difference_ms / 60;
            var minutes = Math.floor(difference_ms % 60);
            difference_ms = difference_ms / 60;
            var hours = Math.floor(difference_ms % 24);
            var days = Math.floor(difference_ms / 24);

            //return (hours < 10 ? '0'+hours:hours) + ':' + (minutes<10 ? '0'+minutes:minutes) + ':' + (seconds<10?'0'+seconds:seconds);
            return (hours < 10 ? '0' + hours : hours) + ':' + (minutes < 10 ? '0' + minutes : minutes);
        }

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This is the part I get the error on - I presume this line is not working  - var duration = diff;  as the related Text Box I am using to display the result doesn't get populated...? Sorry bit out of my depth

 

if (startdate != '' && starthours != '' && startmins != '' && endhours != '' && enddate != '' && endmins != '') {
                
                // WORK OUT DIFF IN MIN
                //var diff = difference;
               
                // SET THE SIGN IF START > END
                var sign = (diff < 0) ? '' : '';

                // WORK WITH ABS VALUE
                diff = Math.abs(diff);

                // SET THE DIFF IN MINUTES
               

                // CREATE A FORMATTED STRING hh:mm
                var hours = parseInt(diff / 60);
                var min = diff - (hours * 60);

                // LETS PAD THE STRING
                min = "00" + min;
                hours = "00" + hours;
                var hoursdecimal = "";
                var hoursrounded = "";
                var duration = diff;
                //var duration = sign + hours.substr(hours.length - 2) + ':' + min.substring(min.length - 2);

                
                if (duration.split(':')[1] >= '00' && duration.split(':')[1] <= '14') {
                    hoursdecimal = duration.split(':')[0] + ".00";
                    hoursrounded = duration.split(':')[0] + ":00";
                }

                if (duration.split(':')[1] >= '15' && duration.split(':')[1] <= '29') {
                    hoursdecimal = duration.split(':')[0] + ".25";
                    hoursrounded = duration.split(':')[0] + ":15";
                }

                if (duration.split(':')[1] >= '30' && duration.split(':')[1] <= '44') {
                    hoursdecimal = duration.split(':')[0] + ".50";
                    hoursrounded = duration.split(':')[0] + ":30";
                }

                if (duration.split(':')[1] >= '45' && duration.split(':')[1] <= '59') {
                    hoursdecimal = duration.split(':')[0] + ".75";
                    hoursrounded = duration.split(':')[0] + ":45";
                }

                // SET THE FORMATTED DIFF
                document.getElementById('<%=TxtDuration.ClientID%>').value = duration;
                // SET THE FORMATTED DIFF
                document.getElementById('<%=TxtDurationRounded.ClientID%>').value = hoursrounded;
                // SET THE FORMATTED DIFF
                document.getElementById('<%=TxtDurationDecimal.ClientID%>').value = hoursdecimal;

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