Don't parse XML/HTML with regex, instead use a proper parser like xmlstarlet with a xpath expression :
Example
File :
<!DOCTYPE html>
<html>
<head>
<title>test</title>
</head>
<body>
<a>foo</a>
<div class="IASBS_Date_en">20180219</div>
<b>bar</b>
<c>base</c>
</body>
</html>
Command :
xmlstarlet edit -L -u '//div[@class="IASBS_Date_en"]' -v '20190101' file.html
Edited file :
<!DOCTYPE html>
<html>
<head>
<title>test</title>
</head>
<body>
<a>foo</a>
<div class="IASBS_Date_en">20190101</div>
<b>bar</b>
<c>base</c>
</body>
</html>
You just have to replace your sed command with this one