abap casting with field symbol type

TYPES: BEGIN OF t_date,
          year(4)  TYPE n,
          month(2) TYPE n,
          day(2)   TYPE n,
       END OF t_date.FIELD-SYMBOLS <fs> TYPE t_date.
ASSIGN sy-datum TO <fs> CASTING.
WRITE / sy-datum.
SKIP.
WRITE: / <fs>-year , / <fs>-month, / <fs>-day.