Please note that if you are using DATE_RFC7231 format (used in HTTP/1.1), you'll need to change the DateTime object timezone to GMT *before*, or you'll encounter weird results, as this format DOES NOT convert the date to GMT.
So if you have a DateTime object using UTC+01:00 as its timezone, you will get a difference of 1 hour between your resulting date string and what should be the "correct" date.
Recommended use:
<?php <span class="katex"><span class="katex-mathml"><math xmlns="http://www.w3.org/1998/Math/MathML"><semantics><mrow><mi>d</mi><mi>a</mi><mi>t</mi><msub><mi>e</mi><mi>g</mi></msub><mi>m</mi><mi>t</mi><mo>=</mo><mi>c</mi><mi>l</mi><mi>o</mi><mi>n</mi><mi>e</mi></mrow><annotation encoding="application/x-tex">date_gmt = clone </annotation></semantics></math></span><span class="katex-html" aria-hidden="true"><span class="base"><span class="strut" style="height:0.9805em;vertical-align:-0.2861em;"></span><span class="mord mathnormal">d</span><span class="mord mathnormal">a</span><span class="mord mathnormal">t</span><span class="mord"><span class="mord mathnormal">e</span><span class="msupsub"><span class="vlist-t vlist-t2"><span class="vlist-r"><span class="vlist" style="height:0.1514em;"><span style="top:-2.55em;margin-left:0em;margin-right:0.05em;"><span class="pstrut" style="height:2.7em;"></span><span class="sizing reset-size6 size3 mtight"><span class="mord mathnormal mtight" style="margin-right:0.03588em;">g</span></span></span></span><span class="vlist-s"></span></span><span class="vlist-r"><span class="vlist" style="height:0.2861em;"><span></span></span></span></span></span></span><span class="mord mathnormal">m</span><span class="mord mathnormal">t</span><span class="mspace" style="margin-right:0.2778em;"></span><span class="mrel">=</span><span class="mspace" style="margin-right:0.2778em;"></span></span><span class="base"><span class="strut" style="height:0.6944em;"></span><span class="mord mathnormal">c</span><span class="mord mathnormal" style="margin-right:0.01968em;">l</span><span class="mord mathnormal">o</span><span class="mord mathnormal">n</span><span class="mord mathnormal">e</span></span></span></span>date;
$date_gmt->setTimezone(new \DateTimeZone('GMT'));
echo $date_gmt->format(DATE_RFC7231);
?>