[AT]
Expects: AH    05H
CH    century, in BCD  (19H ... 20H)
CL    year, in BCD     (00H ... 99H)
DH    month, in BCD    (i.e., 01H=Jan ... 12H=Dec)
DL    day, in BCD      (00H ... 31H)
                                                                  
Returns: AH    0
AL    value written to CMOS 0bH register
CF    NC (0) clock operating
CY (1) clock not operating or busy being updated
                                                                  
Info: Sets the date for the battery-backed CMOS Real-Time Clock.

Input values are in BCD ; for instance, CX = 1998H = 1998.

The BIOS avoids reentrancy by not setting the date if the clock
happens to be in the process of being updated.  Thus, if CF is
set on return, you should try a few more times before giving up.

See Also: INT 1aH (BIOS Time I/O)
DOS fn 2bH (set system date)
- -

INT 1aH 05H: Set Date on Real-Time Clock