Explain TIMESTAMP, INTERVAL YEAR TO MONTH and INTERVAL DAY TO SECOND

TIMESTAMP(n) adds n digits of fractional seconds. TIMESTAMP WITH TIME ZONE stores an explicit offset; TIMESTAMP WITH LOCAL TIME ZONE normalises to the DB zone on store and converts to the session zone on read. Subtracting two timestamps yields an INTERVAL DAY TO SECOND. INTERVAL YEAR TO MONTH stores year/month durations. Build literals with INTERVAL '2' HOUR, INTERVAL '1-6' YEAR TO MONTH, or NUMTODSINTERVAL / NUMTOYMINTERVAL.

DECLARE
  v_start TIMESTAMP := SYSTIMESTAMP;
  v_gap   INTERVAL DAY TO SECOND;
BEGIN
  v_gap := SYSTIMESTAMP - v_start;                 -- a duration
  DBMS_OUTPUT.PUT_LINE('elapsed: ' || v_gap);
  DBMS_OUTPUT.PUT_LINE('in 90 min: ' ||
    (SYSTIMESTAMP + NUMTODSINTERVAL(90, 'MINUTE')));
END;
/