Daylight saving time vs tsz timestamp not consistent.

I have a PLS application that create a date and a 'tsz' timestamp from two numeric values: Date portion and time portion.
.......... call to the convert function........
      IF bOk THEN
        -- Compile a begin date
        bOk := Disturb_lib.ConvInt2DT(in_iBeginDate,in_iBeginTime,dtBeginDT,sErrMsg);
      END IF;
      IF bOk THEN
        -- Compile a begin timestamp
        bOk := Disturb_lib.ConvInt2TSZ(in_iBeginDate,in_iBeginTime,tmpBeginTsz,sErrMsg );
      END IF;

........... and the convert function..............
FUNCTION ConvInt2TSZ( in_iDatePart            IN INTEGER, 
                      in_iTimePart            IN INTEGER,
                      out_tszDate             OUT TIMESTAMP WITH TIME ZONE,
                      out_sErrMsg             OUT VARCHAR2
                  ) RETURN BOOLEAN

....... validation........

      out_tszDate := TO_TIMESTAMP( TO_CHAR( in_iDatePart*1000000 ) + in_iTimePart, 'YYYYMMDDHH24MISS' );