diff --git a/actions/favorited.php b/actions/favorited.php
index 150b67b0b0..9ffa5b8445 100644
--- a/actions/favorited.php
+++ b/actions/favorited.php
@@ -185,11 +185,7 @@ class FavoritedAction extends Action
 
     function showContent()
     {
-        if (common_config('db', 'type') == 'pgsql') {
-            $weightexpr='sum(exp(-extract(epoch from (now() - fave.modified)) / %s))';
-        } else {
-            $weightexpr='sum(exp(-(now() - fave.modified) / %s))';
-        }
+        $weightexpr = common_sql_weight('fave.modified', common_config('popular', 'dropoff'));
 
         $qry = 'SELECT notice.*, '.
           $weightexpr . ' as weight ' .
@@ -207,7 +203,7 @@ class FavoritedAction extends Action
         }
 
         $notice = Memcached_DataObject::cachedQuery('Notice',
-                                                    sprintf($qry, common_config('popular', 'dropoff')),
+                                                    $qry,
                                                     600);
 
         $nl = new NoticeList($notice, $this);
diff --git a/actions/publictagcloud.php b/actions/publictagcloud.php
index 5c70740290..b5b474f132 100644
--- a/actions/publictagcloud.php
+++ b/actions/publictagcloud.php
@@ -105,12 +105,8 @@ class PublictagcloudAction extends Action
 
         #Add the aggregated columns...
         $tags->selectAdd('max(notice_id) as last_notice_id');
-        if(common_config('db','type')=='pgsql') {
-            $calc='sum(exp(-extract(epoch from (now()-created))/%s)) as weight';
-        } else {
-            $calc='sum(exp(-(now() - created)/%s)) as weight';
-        }
-        $tags->selectAdd(sprintf($calc, common_config('tag', 'dropoff')));
+        $calc = common_sql_weight('created', common_config('tag', 'dropoff'));
+        $tags->selectAdd($calc . ' as weight');
         $tags->groupBy('tag');
         $tags->orderBy('weight DESC');
 
@@ -136,10 +132,11 @@ class PublictagcloudAction extends Action
             $this->elementStart('dd');
             $this->elementStart('ul', 'tags xoxo tag-cloud');
             foreach ($tw as $tag => $weight) {
+                common_log(LOG_DEBUG, "$weight/$sum");
                 if ($sum) {
                     $weightedSum = $weight/$sum;
                 } else {
-                    $weightedSum = 1;
+                    $weightedSum = 0.5;
                 }
                 $this->showTag($tag, $weight, $weightedSum);
             }
diff --git a/lib/grouptagcloudsection.php b/lib/grouptagcloudsection.php
index 091cf48457..14ceda0850 100644
--- a/lib/grouptagcloudsection.php
+++ b/lib/grouptagcloudsection.php
@@ -58,11 +58,7 @@ class GroupTagCloudSection extends TagCloudSection
 
     function getTags()
     {
-        if (common_config('db', 'type') == 'pgsql') {
-            $weightexpr='sum(exp(-extract(epoch from (now() - notice_tag.created)) / %s))';
-        } else {
-            $weightexpr='sum(exp(-(now() - notice_tag.created) / %s))';
-        }
+        $weightexpr = common_sql_weight('notice_tag.created', common_config('tag', 'dropoff'));
 
         $names = $this->group->getAliases();
 
@@ -99,7 +95,6 @@ class GroupTagCloudSection extends TagCloudSection
 
         $tag = Memcached_DataObject::cachedQuery('Notice_tag',
                                                  sprintf($qry,
-                                                         common_config('tag', 'dropoff'),
                                                          $this->group->id,
                                                          $namestring),
                                                  3600);
diff --git a/lib/personaltagcloudsection.php b/lib/personaltagcloudsection.php
index 0b29d58ca6..091425f926 100644
--- a/lib/personaltagcloudsection.php
+++ b/lib/personaltagcloudsection.php
@@ -58,13 +58,9 @@ class PersonalTagCloudSection extends TagCloudSection
 
     function getTags()
     {
-        if (common_config('db', 'type') == 'pgsql') {
-            $weightexpr='sum(exp(-extract(epoch from (now() - notice_tag.created)) / %s))';
-        } else {
-            $weightexpr='sum(exp(-(now() - notice_tag.created) / %s))';
-        }
- 
-       $qry = 'SELECT notice_tag.tag, '.
+        $weightexpr = common_sql_weight('notice_tag.created', common_config('tag', 'dropoff'));
+
+        $qry = 'SELECT notice_tag.tag, '.
           $weightexpr . ' as weight ' .
           'FROM notice_tag JOIN notice ' .
           'ON notice_tag.notice_id = notice.id ' .
@@ -83,7 +79,6 @@ class PersonalTagCloudSection extends TagCloudSection
 
         $tag = Memcached_DataObject::cachedQuery('Notice_tag',
                                                  sprintf($qry,
-                                                         common_config('tag', 'dropoff'),
                                                          $this->user->id),
                                                  3600);
         return $tag;
diff --git a/lib/popularnoticesection.php b/lib/popularnoticesection.php
index 9fbc9d2ddd..fbf9a60ab8 100644
--- a/lib/popularnoticesection.php
+++ b/lib/popularnoticesection.php
@@ -48,17 +48,17 @@ class PopularNoticeSection extends NoticeSection
 {
     function getNotices()
     {
+        // @fixme there should be a common func for this
         if (common_config('db', 'type') == 'pgsql') {
-            $weightexpr='sum(exp(-extract(epoch from (now() - fave.modified)) / %s))';
             if (!empty($this->out->tag)) {
                 $tag = pg_escape_string($this->out->tag);
             }
         } else {
-            $weightexpr='sum(exp(-(now() - fave.modified) / %s))';
             if (!empty($this->out->tag)) {
                  $tag = mysql_escape_string($this->out->tag);
             }
         }
+        $weightexpr = common_sql_weight('fave.modified', common_config('popular', 'dropoff'));
         $qry = "SELECT notice.*, $weightexpr as weight ";
         if(isset($tag)) {
             $qry .= 'FROM notice_tag, notice JOIN fave ON notice.id = fave.notice_id ' .
@@ -78,7 +78,7 @@ class PopularNoticeSection extends NoticeSection
         $qry .= ' LIMIT ' . $limit . ' OFFSET ' . $offset;
 
         $notice = Memcached_DataObject::cachedQuery('Notice',
-                                                    sprintf($qry, common_config('popular', 'dropoff')),
+                                                    $qry,
                                                     1200);
         return $notice;
     }
diff --git a/lib/util.php b/lib/util.php
index 63656b6042..50bd0e2ac9 100644
--- a/lib/util.php
+++ b/lib/util.php
@@ -908,6 +908,26 @@ function common_sql_date($datetime)
     return strftime('%Y-%m-%d %H:%M:%S', $datetime);
 }
 
+/**
+ * Return an SQL fragment to calculate an age-based weight from a given
+ * timestamp or datetime column.
+ *
+ * @param string $column name of field we're comparing against current time
+ * @param integer $dropoff divisor for age in seconds before exponentiation
+ * @return string SQL fragment
+ */
+function common_sql_weight($column, $dropoff)
+{
+    if (common_config('db', 'type') == 'pgsql') {
+        // PostgreSQL doesn't support timestampdiff function.
+        // @fixme will this use the right time zone?
+        // @fixme does this handle cross-year subtraction correctly?
+        return "sum(exp(-extract(epoch from (now() - $column)) / $dropoff))";
+    } else {
+        return "sum(exp(timestampdiff(second, utc_timestamp(), $column) / $dropoff))";
+    }
+}
+
 function common_redirect($url, $code=307)
 {
     static $status = array(301 => "Moved Permanently",