JSFiddle - React, Tailwind, and code Playground

HTML

<table>
    <thead>
        <tr>
            <td>Name</td>
            <td>Startdate</td>
            <td>Date in years</td>
        </tr>
    </thead>
    <tbody>
        <tr>
            <td>Employee #1</td>
            <td>1992/07/07 18:00:00</td>
        </tr>
        <tr>
            <td>Employee #2</td>
            <td>1990/01/01 18:00:00</td>
        </tr>
        <tr>
            <td>Employee #3</td>
            <td>2013/01/01 18:00:00</td>
        </tr>
        <tr>
            <td>Employee #4</td>
            <td>2012/01/01 18:00:00</td>
        </tr>
    </tbody>
</table>

CSS

td { padding:2px; border:1px solid black; }

JavaScript

var rows = document.querySelectorAll("tbody tr"); // Get all the rows

for(var i = 0; i < rows.length; i++) {
    // For each row, get the start date
    var startdate = rows[i].cells[1].innerText,
        years = calculateYears(startdate);
	
    // Create a DOM element with the result, and add it to the table
    var td = document.createElement("td"),
    	result = document.createTextNode(years);
    td.appendChild(result);
    rows[i].appendChild(td);
}

// Years calculation
function calculateYears(startdate) {
    var dateObj1 = new Date( startdate );
    var dateObj2 = new Date();
    
    //get difference in milliseconds
    var diffMilliseconds = dateObj1.getTime() - dateObj2.getTime();
    
    //make the difference positive
    if( diffMilliseconds < 0 ) diffMilliseconds *= -1;
    
    //convert milliseconds to hours
    var diffYears = ( diffMilliseconds / 1000 ) / 60 / 60 / 24 / 365;
    
    //print on console
    var num = diffYears;
    num = Math.floor(num);
    
    if (num > 1)
    {
    return num + ' years';
    }
    else if (num < 1)
    {
    return 'less than one year';
    }
    else
    {
    return num + ' year';
    }
}