diff --git a/build/uk_master_export.xsl b/build/uk_master_export.xsl
index 9fecb470..9ae5cab6 100644
--- a/build/uk_master_export.xsl
+++ b/build/uk_master_export.xsl
@@ -19,19 +19,55 @@
 	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 	xmlns:wayf="http://sdss.ac.uk/2006/06/WAYF"
 	xmlns:uklabel="http://ukfederation.org.uk/2006/11/label"
+
 	xmlns:date="http://exslt.org/dates-and-times"
+	xmlns:mdxDates="xalan://uk.ac.sdss.xalan.md.Dates"
+	extension-element-prefixes="date mdxDates"
+
 	xmlns="urn:oasis:names:tc:SAML:2.0:metadata"
 	exclude-result-prefixes="wayf">
 
 	<!--Force UTF-8 encoding for the output.-->
 	<xsl:output omit-xml-declaration="no" method="xml" encoding="UTF-8" indent="yes"/>
 
+	<!--
+		validityDays
+		
+		This parameter determines the number of days between the aggregation instant and the
+		end of validity of the signed metadata.
+	-->
+	<xsl:variable name="validityDays" select="14"/>
+	
 	<xsl:variable name="now" select="date:date-time()"/>
+	<xsl:variable name="validUntil" select="mdxDates:dateAdd($now, $validityDays)"/>
 	
 	<!--
-		Add a comment to the start of the output file.
+		Document root.
 	-->
 	<xsl:template match="/">
+		<xsl:call-template name="document.comment"/>
+		<xsl:apply-templates/>
+	</xsl:template>
+	
+	<!--
+		Document element.
+	-->
+	<xsl:template match="/md:EntitiesDescriptor">
+		<xsl:copy>
+			<xsl:attribute name="validUntil">
+				<xsl:value-of select="$validUntil"/>
+			</xsl:attribute>
+			<xsl:apply-templates select="@*"/>
+			<xsl:call-template name="document.comment"/>
+			<xsl:apply-templates select="node()"/>
+		</xsl:copy>
+	</xsl:template>
+	
+	<!--
+		Comment to be added to the top of the document, and just inside the document element.
+	-->
+	<xsl:template name="document.comment">
+		<xsl:text>&#10;</xsl:text>
 		<xsl:comment>
 			<xsl:text>&#10;&#9;U K   F E D E R A T I O N   M E T A D A T A&#10;</xsl:text>
 			<xsl:text>&#10;</xsl:text>
@@ -40,10 +76,25 @@
 			<xsl:text>&#9;Aggregate built </xsl:text>
 			<xsl:value-of select="$now"/>
 			<xsl:text>&#10;</xsl:text>
+			<xsl:text>&#10;</xsl:text>
+			<xsl:text>&#9;Aggregate valid for </xsl:text>
+			<xsl:value-of select="$validityDays"/>
+			<xsl:text> days, until </xsl:text>
+			<xsl:value-of select="$validUntil"/>
+			<xsl:text>&#10;</xsl:text>
 		</xsl:comment>
-		<xsl:apply-templates/>
+		<xsl:text>&#10;</xsl:text>
 	</xsl:template>
-	
+	
+	<!--
+		Drop text nodes inside the document element.  There's one of these for each
+		EntityDescriptor in the original document, so without this most of the output
+		file is blank.
+	-->
+	<xsl:template match="md:EntitiesDescriptor/text()">
+		<!-- do nothing -->
+	</xsl:template>
+	
 	<!--
 		Drop the federation-specific key authority information.