<xsl:for-each select="//magadvert">
    <xsl:if test="releaseDate > 2018-09-15">
        <tr>
            ...Do Something
        </tr>
    </xsl:if>
</xsl:for-each>

<magadvert advertID="1">
    <releaseDate>2018-06-22</releaseDate>
    <endDate>2018-09-08</endDate>
    <magName>Phasellus At Augue Industries</magName>
    <advertSize>3971.69</advertSize>
    <position>Inside front</position>
    <magCost>35217.49</magCost>
    <issuesAppeared>11</issuesAppeared>
</magadvert>