The basic idea is that every time somebody visits the website the cookie saves the time and date. If the user then visits the website multiple times the cookie only shows the date and time when the user last visited. At the moment I have the following code which does work put obviously always updates the date and time:

var today = new Date();
var date = today.getDate()+'-'+(today.getMonth()+1)+'-'+today.getFullYear();
var time = today.getHours()+':'+today.getMinutes()+':'+today.getSeconds();
var dateTime = date+' at '+time;
document.cookie ='last_visit='+ dateTime 

Edit:For other people who might need help, the following code seems to work:

var today = new Date();
var date = today.getDate()+'-'+(today.getMonth()+1)+'-'+today.getFullYear();
var time = today.getHours()+':'+today.getMinutes()+':'+today.getSeconds();
var dateTime = date+' at '+time;
function getCookieValue(a) {
    const b = document.cookie.match('(^|;)\\s*' + a + '\\s*=\\s*([^;]+)');
    return b ? b.pop() : '';
}
console.log ("getCookieValue " + getCookieValue("last_visit"));
document.cookie ='last_visit='+ dateTime