This will give you the result.
$html = str_get_html('<div class="parent" id="parent">
<div class="child">
<div class="grandchild">
<p>Text Paragraph</p>
<h2>Heading</h2>
<ul>
<li>Heading Date Text</li>
<li>Heading Date Text</li>
<li>Heading Date Text</li>
<li>Heading Date Text</li>
<li>Heading Date Text</li>
</ul>
<p>Text</p>
</div>
</div>
</div>');
foreach($html->find('div#parent ul') as $ul)
{
foreach($ul->find('li') as $li)
{
echo $li->plaintext."<br>";
}
}