[AT]
Expects: AH    03H
CH    hours, in BCD 
CL    minutes, in BCD
DH    seconds, in BCD
DL    00h = no Daylight Savings Time option
01h = Daylight Savings Time option
                                                                  
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 time for the battery-backed CMOS Real-Time Clock.

All input values are in BCD  in 24-hour format; for instance,
CX = 1234H = 12:34pm.

The BIOS avoids reentrancy by not allowing the time to be set 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 2dH (set system time)
- -

INT 1aH 03H: Set Time on Real-Time Clock