Display 5 digit Julian Date in IFRAME

I have a locally hosted (on my machine) simple HTML page that we use in the office for tracking work orders. I made it as simple as possible. One dynamic element I need is to display the 5 digit Julian date inside of an IFRAME.

The format needs to be (ex): 24043
24 for the last two of the year, and 043 being the 43rd day of this calendar year (February 12, 2024).

I have tried multiple different posts, and guides to get this working, but none of them thus far has worked correctly, or most times, at all.

Here is the code I have in my HEAD at the moment:

<script type="text/javascript">
document.getElementById('txt').julianDate;
//set an array with day counts for all months
var dayCount = [31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31];

function convertDate(str) {
   //convert passed string to date object
   var dte = new Date(str);
   //initialize date variable
   var julianDate = 0;
   //add days for previous months
   for (i = 0; i < dte.getMonth(); i++) {
      julianDate += dayCount[i];
   }
   //add days of the current month
   julianDate += dte.getDate();
   //check for leap year
   if (dte.getFullYear() % 4 == 0 && dte.getMonth() > 1) {
      julianDate++;
   }
   //alert("Julian Date: " + julianDate);
   //var jdate = julianDate
   //return julianDate;
   document.getElementById("JulianDate").innerHTML = julianDate2
}
</script>

And in the body (as a test) I have this:

<h1>"The value for number is: " <span id="JulianDate2"></span></h1>

My current output is:

“The value for number is: “

with no value being put out. I’m not a programming whiz by any means, so I have to look most of this up, and work my way though it the best I can.