Int 21 Fn Sect  - Dos 1+ - Set System Date                                 [!]

   AH = 2Bh
   CX = year (1980-2099)
   DH = month
   DL = day

Return: AL = status
       00h successful
       FFh invalid date, system date unchanged

Note:  DOS 3.3+ also sets CMOS clock

See Also: AH=2Ah,AH=2Dh,INT 1A/AH=05h