JS/CSS - customizing trustpilot widget

I am trying to modify the trustpilot widget on my site.

The basic structure of the trustpilot widget is:

#tp-widget_wrapper .tp-widget-wrapper
    #wrapper-top .wrapper-top
        #tp-widget-reviews-wrapper .tp-widget-reviews-wrapper hidden-element
            #tp-widget-reviews .tp-widget-reviews
                .tp-widget-review
                    .tp-widget-stars
                    .date

I am trying to hide the div .date.

What I have gotten so far is:

var trustpilot_children = document.getElementById("trustpilot_widget").childNodes;
var trustpilot_frame = trustpilot_children[0];
var date_tags = trustpilot_frame.document.getElementsByClassName("date");
date_tags.style.display = "none";

However I get the error:

Uncaught TypeError: Cannot read property 'getElementsByClassName' of undefined

The var trustpilot_frame is finding the iframe, I just can't access anything within it.

Any help or suggestions with this would be greatly appreciated.

//-----EDIT-----//

var trustpilot_children = document.getElementById("trustpilot_widget").childNodes;
var trustpilot_frame = trustpilot_children[0].contentDocument;
console.log(trustpilot_frame);
var date_tags = trustpilot_frame.getElementsByClassName("date");
console.log(date_tags);
date_tags.style.display = "none";

Console:

//-----EDIT2-----//

var trustpilot_children = document.getElementById("trustpilot_widget").childNodes;
var trustpilot_frame = trustpilot_children[0].contentDocument;
var style_tag = trustpilot_frame.createElement("style");
style_tag.textContent = ".date{display:none;}";
trustpilot_frame.getElementsByTagName("head")[0].appendChild(style_tag);
console.log(trustpilot_frame);

Console:

//-----EDIT3-----//

var trustpilot_children = document.getElementById("trustpilot_widget").childNodes;
var trustpilot_frame = trustpilot_children[0].contentDocument;
trustpilot_frame.onload = setTimeout(hide_dates, 10000);
function hide_dates(){

    // method 1
    var style_tag = trustpilot_frame.createElement("style");
    style_tag.textContent = ".date{display:none;}";
    trustpilot_frame.getElementsByTagName("head")[0].appendChild(style_tag);
    console.log(trustpilot_frame);

    // method 2
    var date_tags = trustpilot_frame.getElementsByClassName("date");
    console.log(date_tags);
    date_tags.style.display = "none";
}

Console shows the same as EDIT2 for the dom and the first EDIT for date_tags

https://codepen.io/phallihan/pen/OdwgGd

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