Oracle 11g Release 2

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ALTER SESSION SET NLS_DATE_FORMAT = 'YYYY-MM-DD HH24:MI:SS';
ALTER SESSION SET NLS_TIMESTAMP_TZ_FORMAT = 'YYYY-MM-DD HH24:MI:SS.FF9';
SELECT TRUNC( current_timestamp, 'MI' )
         - MOD( EXTRACT( MINUTE FROM current_timestamp ), 15 ) * INTERVAL '1' MINUTE
         - INTERVAL '30' MINUTE
         + INTERVAL '59' SECOND AS start_time,
       TRUNC( current_timestamp, 'MI' )
         - MOD( EXTRACT( MINUTE FROM current_timestamp ), 15 ) * INTERVAL '1' MINUTE
         - INTERVAL '15' MINUTE
         + INTERVAL '59' SECOND AS end_time,
       current_timestamp
FROM   DUAL


START_TIME
END_TIME
CURRENT_TIMESTAMP


2024-06-26 15:30:59
2024-06-26 15:45:59
2024-06-26 16:06:45.928155000