diff --git a/qa/common b/qa/common
index a3d0090..532343c 100755
--- a/qa/common
+++ b/qa/common
@@ -18,6 +18,7 @@ _setenvironment()
     MSGVERB="text:action"
     export MSGVERB
     export PCP_BINADM_DIR
+    export PCP_DERIVED_CONFIG=""
 }
 
 _log_fyi()
diff --git a/src/pcp/iostat/pcp-iostat.py b/src/pcp/iostat/pcp-iostat.py
index ba0c5c4..90e1c68 100755
--- a/src/pcp/iostat/pcp-iostat.py
+++ b/src/pcp/iostat/pcp-iostat.py
@@ -40,9 +40,9 @@ IOSTAT_DM_METRICS = [ 'disk.dm.read', 'disk.dm.read_bytes',
 class IostatReport(pmcc.MetricGroupPrinter):
     Hcount = 0
     def timeStampDelta(self, group):
-        c = 1000000.0 * group.timestamp.tv_sec + group.timestamp.tv_usec
-        p = 1000000.0 * group.prevTimestamp.tv_sec + group.prevTimestamp.tv_usec
-        return (c - p) / 1000000.0
+        s = group.timestamp.tv_sec - group.prevTimestamp.tv_sec
+        u = group.timestamp.tv_usec - group.prevTimestamp.tv_usec
+        return (s + u / 1000000.0)
 
     def instlist(self, group, name):
         return dict(map(lambda x: (x[1], x[2]), group[name].netValues)).keys()