SQL> with
  2    data as (
  3      select to_timestamp_tz('2015-09-11 Asia/Calcutta','YYYY-MM-DD TZR') dt from dual
  4    )
  5  select dt, dt at time zone 'UTC' utcdt, dt at time zone 'GMT' gmtdt
  6  from data
  7  /
DT                                    UTCDT                       GMTDT
------------------------------------- --------------------------- ---------------------------
11/09/2015 00:00:00.000 ASIA/CALCUTTA 10/09/2015 18:30:00.000 UTC 10/09/2015 18:30:00.000 GMT