diff --git a/mdx/common-beans.xml b/mdx/common-beans.xml
index 16387525..b2cf8ab4 100644
--- a/mdx/common-beans.xml
+++ b/mdx/common-beans.xml
@@ -154,7 +154,32 @@
         Approximately when the Spring context was initialised,
         as a java.time.Instant.
     -->
-    <bean id="now_instant_raw" parent="Instant" factory-method="now"/>
+    <bean id="start_instant_raw" parent="Instant" factory-method="now"/>
+
+    <!--
+        A java.time.Instant representing 00:00:00Z today.
+
+        We use this to pin timestamps in aggregates intended for comparison.
+    -->
+    <bean id="today_instant_raw" parent="Instant" factory-method="from"
+        c:_="#{ start_instant_raw.truncatedTo(T(java.time.temporal.ChronoUnit).DAYS) }"/>
+
+    <!--
+        Select the instant to use as "now" in places like aggregate generation.
+
+        By default, this will be the time the Spring context was initialised.
+        This may be overridden in order to provide a stable value for
+        run-to-run comparisons.
+
+        Set the now.instant.raw.name to the name of the overriding value bean.
+        If running from build.xml using ant, this needs to be prefixed by "mda.".
+
+        Example:
+
+            ant -Dmda.now.instant.raw.name=today_instant_raw ...
+    -->
+    <bean id="now_instant_raw" parent="Instant" factory-method="from"
+        c:_-ref="${now.instant.raw.name:start_instant_raw}"/>
 
     <!--
         now_instant_raw can have any precision. To get somewhat consistent
@@ -180,6 +205,22 @@
         c:_1="#{ T(java.time.ZoneId).systemDefault() }"/>
     <bean id="now_local_ISO" parent="String" c:_="#{ now_local.toString() }"/>
 
+    <!--
+        Make a value for the validUntil attribute for any aggregates we
+        generate.
+
+        This is based on the now_instant plus a number of days specified
+        by the validUntil.aggregate.days property (or 14 days if for some reason
+        that is not defined).
+
+        Assumes that a day is exactly 86400 seconds, which is close enough for
+        these purposes.
+    -->
+    <bean id="validuntil_aggregate_days" class="java.lang.Integer" factory-method="parseInt"
+        c:_="${validUntil.aggregate.days:14}"/>
+    <bean id="validuntil_aggregate_ISO" parent="String"
+        c:_="#{ now_instant.plusSeconds(86400*validuntil_aggregate_days).toString()}"/>
+
     <!--
         ***********************************************
         ***                                         ***
diff --git a/mdx/uk/final_tweak.xsl b/mdx/uk/final_tweak.xsl
index e6c30a44..5a800ce0 100644
--- a/mdx/uk/final_tweak.xsl
+++ b/mdx/uk/final_tweak.xsl
@@ -62,17 +62,27 @@
     -->
     <xsl:param name="now_local_ISO"/>
 
+    <!--
+        valid_until_ISO
+
+        This parameter is an ISO8601 representation of the UTC instant
+        at which the aggregate will become invalid. This has been computed
+        by the caller as now_ISO + validityDays days.
+    -->
+    <xsl:param name="valid_until_ISO"/>
+
     <!--
         Document element.
     -->
     <xsl:template match="/md:EntitiesDescriptor">
         <xsl:call-template name="document.comment">
-            <xsl:with-param name="validUntil" select="@validUntil"/>
+            <xsl:with-param name="validUntil" select="$valid_until_ISO"/>
         </xsl:call-template>
         <EntitiesDescriptor>
             <xsl:apply-templates select="@*"/>
+            <xsl:attribute name="validUntil"><xsl:value-of select="$valid_until_ISO"/></xsl:attribute>
             <xsl:call-template name="document.comment">
-                <xsl:with-param name="validUntil" select="@validUntil"/>
+                <xsl:with-param name="validUntil" select="$valid_until_ISO"/>
             </xsl:call-template>
 
             <!--
diff --git a/mdx/uk/generate.xml b/mdx/uk/generate.xml
index 3784bfea..dc5963f5 100644
--- a/mdx/uk/generate.xml
+++ b/mdx/uk/generate.xml
@@ -85,13 +85,12 @@
     <!--
         assembleAggregate
 
-        Assemble a UK-style aggregate with an appropriate validUntil and ID.
+        Assemble a UK-style aggregate with an appropriate ID.
     -->
     <bean id="assembleAggregate" parent="mda.CompositeStage">
         <property name="composedStages">
             <list>
                 <ref bean="uk_assemble"/>
-                <bean parent="mda.SetValidUntilStage" p:validityDuration="${validUntil.aggregate.duration}"/>
                 <!--
                     setStaticID
 
@@ -282,6 +281,7 @@
                 <entry key="validityDays" value="${validUntil.aggregate.days}"/>
                 <entry key="now_ISO" value-ref="now_ISO"/>
                 <entry key="now_local_ISO" value-ref="now_local_ISO"/>
+                <entry key="valid_until_ISO" value-ref="validuntil_aggregate_ISO"/>
             </map>
         </property>
     </bean>
@@ -438,6 +438,7 @@
                 <entry key="validityDays" value="${validUntil.aggregate.days}"/>
                 <entry key="now_ISO" value-ref="now_ISO"/>
                 <entry key="now_local_ISO" value-ref="now_local_ISO"/>
+                <entry key="valid_until_ISO" value-ref="validuntil_aggregate_ISO"/>
             </map>
         </property>
     </bean>
@@ -522,6 +523,7 @@
                 <entry key="validityDays" value="${validUntil.aggregate.days}"/>
                 <entry key="now_ISO" value-ref="now_ISO"/>
                 <entry key="now_local_ISO" value-ref="now_local_ISO"/>
+                <entry key="valid_until_ISO" value-ref="validuntil_aggregate_ISO"/>
             </map>
         </property>
     </bean>
@@ -581,6 +583,7 @@
                 <entry key="validityDays" value="${validUntil.aggregate.days}"/>
                 <entry key="now_ISO" value-ref="now_ISO"/>
                 <entry key="now_local_ISO" value-ref="now_local_ISO"/>
+                <entry key="valid_until_ISO" value-ref="validuntil_aggregate_ISO"/>
             </map>
         </property>
     </bean>
@@ -638,6 +641,7 @@
                 <entry key="validityDays" value="${validUntil.aggregate.days}"/>
                 <entry key="now_ISO" value-ref="now_ISO"/>
                 <entry key="now_local_ISO" value-ref="now_local_ISO"/>
+                <entry key="valid_until_ISO" value-ref="validuntil_aggregate_ISO"/>
             </map>
         </property>
     </bean>