How to calculate cleaning turns if every flatmate is doing a two weeks turn dynamically?

Hi all,

I am building a table that automatically shows who is the person that is in charge of cleaning the kitchen in plain Javascript.

This is what I have so far:

<style>
th, td {
  padding: 15px;
  text-align: left;
}

tr:hover {background-color: #f5f5f5;}

table {
  width: 100%;
}
</style>

<div id="cleaningTableDiv">
</div>


<script>

Date.prototype.getWeek = function() {
    var onejan = new Date(this.getFullYear(), 0, 1);
    return Math.ceil((((this - onejan) / 86400000) + onejan.getDay() + 1) / 7);
}

function plainTableCreate(currentWeekNumber) {
    // Table variables
    var teamMembers = ["Flatmate1", "Flatmate2", "Flatmate3", "Flatmate4", "Flatmate5", "Flatmate6"];
    var rowNumber = 7;
    var colNumber = teamMembers.length * 2 + 1;
    var headings = ["Week number"]
    for (var i = 0; i < colNumber - 1; i++) {
        headings.push(currentWeekNumber + i);
    }

    // Creating table
    var body = document.getElementById('cleaningTableDiv');
    var tbl = document.createElement('table');
    tbl.setAttribute("id", "cleaningTable");
    var tbdy = document.createElement('tbody');
    for (var i = 0; i < rowNumber; i++) {
        var tr = document.createElement('tr');
        for (var j = 0; j < colNumber; j++) {
            var td = document.createElement('td');
            td.appendChild(document.createTextNode(""));
            tr.appendChild(td);
        }
        tbdy.appendChild(tr);
    }
    tbl.appendChild(tbdy);
    body.appendChild(tbl);

    // Populating headings
    for (var i=0; i < headings.length; i++){
        document.getElementById("cleaningTable").rows[0].cells[i].innerHTML = headings[i];
    }

    // Populating 1st column with Team Members
    for (var i=1; i < rowNumber; i++){
        document.getElementById("cleaningTable").rows[i].cells[0].innerHTML = teamMembers[i-1];
    }

    // Populating X
    for (var i=1; i < rowNumber; i++){
        for (var j=1; j < colNumber; j++){
            if (i == 4) {
                // Flatmate 4 started cleaning week 1 and 2, next rotation will happen every 2
                // weeks, for example flatmate 5 will clean week 3 and 4, flatmate 6 will clean
                // week 5 and 6, flatmate 6 will clean week 7 and 8, flatmate 1 will clean
                // week 9 and 10 and so on...
                document.getElementById("cleaningTable").rows[i].cells[j].innerHTML = "X";                  
            }
            console.log(document.getElementById("cleaningTable").rows[0].cells[j].innerHTML);

        }
    }

}


var currentWeekNumber = (new Date()).getWeek();
plainTableCreate(currentWeekNumber);

</script>

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I cannot think how to populate dynamically the turns, given the fact that the flatmate4 started cleaning week 1 and 2 of the year.

How can I be able to make the script perpetually work?

Thanks for your help.