Convert date to internal format

With 7.40 :

DATA : lw_date_ext TYPE char10,
       lw_date TYPE datum.

lw_date = |{ lw_date_ext+6(4) }{ lw_date_ext+3(2) }{ lw_date_ext(2) }|.

With FM :

DATA : lw_date_ext TYPE char10,
       lw_date     TYPE datum.

  CALL FUNCTION 'CONVERT_DATE_TO_INTERNAL'
    EXPORTING
      date_external = lw_date_ext    " ==> 10.10.2017
    IMPORTING
      date_internal = lw_date        " ==> 20171010
    EXCEPTIONS
      date_external_is_invalid = 1.

See also Convert date to external format.