SELECT begin.snap_id,
         dh_snapshot.begin_interval_time,
         begin.stat_name,
         begin.value, 
         end.value,  -- microseconds
         round ( (end.value - begin.value) / 1000000, 2) delta_sec,
         round ( (end.value - begin.value) / 1000000 / 60 / 60, 2) Act_Aver_Sess
    FROM dba_hist_sys_time_model begin,
         dba_hist_sys_time_model end,
         dba_hist_snapshot     dh_snapshot
   WHERE     begin.snap_id = end.snap_id - 1
         AND begin.stat_name = end.stat_name
         AND begin.stat_name = 'DB time'
         AND dh_snapshot.snap_id = begin.snap_id
ORDER BY snap_id DESC;

52462	01/08/2017 06:00:11,452	DB time	559071642227	568366372299	9294,73	2,58

  SELECT begin.snap_id,
         dh_snapshot.begin_interval_time,
         begin.stat_name,
         begin.value, end.value,
         round ( (end.value - begin.value) / 100, 2) delta_sec,
         round ( (end.value - begin.value) / (100 * 3600), 1)
            activeaveragesessions -- For DB time - centiseconds, assumes hourly snapshots
    FROM dba_hist_snapshot dh_snapshot,
         dba_hist_sysstat begin,
         dba_hist_sysstat end
   WHERE     dh_snapshot.snap_id = begin.snap_id
         AND begin.stat_name = 'DB time'
         AND end.stat_name = begin.stat_name
         AND begin.snap_id = end.snap_id - 1
ORDER BY begin.snap_id desc;

52462	01/08/2017 06:00:11,452	DB time	113917564	115764512	18469,48	5,1