Shorten time interval to one month for indiviudal impressions to be discarded, a=chris

diff --git a/src/models/ImpressionModel.php b/src/models/ImpressionModel.php
index c4051cba9..7e08abf26 100644
--- a/src/models/ImpressionModel.php
+++ b/src/models/ImpressionModel.php
@@ -221,9 +221,11 @@ class ImpressionModel extends Model
             $condition = "UPDATE_PERIOD = $period AND UPDATE_TIMESTAMP >= ?";
             $sum = "SUM(NUM_VIEWS)";
         }
-        // delete user data older than one year
+        /* delete user summary data older than one year, delete impression
+           info more than a month old
+         */
         $sql = "DELETE FROM ITEM_IMPRESSION WHERE VIEW_DATE < ?";
-        $db->execute($sql, [$timestamp]);
+        $db->execute($sql, [$timestamps[C\ONE_MONTH]]);
         $sql = "DELETE FROM ITEM_IMPRESSION_SUMMARY WHERE UPDATE_TIMESTAMP < ?".
             " AND UPDATE_PERIOD > ". C\FOREVER;
         $db->execute($sql, [$timestamp]);