Fix: Use value of the first entry instead of the current time. (Redmine issue: #7600)

lib/mentat/stats/rrd.py

@@ -608,7 +608,7 @@ class RrdStats:
         if not tst:
             tst = int(time.time())
 
-        (rrddb, flag_new) = self.prepare_db(ds_id)
+        (rrddb, flag_new) = self.prepare_db(ds_id, tst - (tst % self.step))
 
         try:
             rrdtool.update(