AH = 2Bh
        DL = day
        DH = month
        CX = year
Return: AL = 0 if no error
        AL = 0FFh if bad value sent to routine
        (DOS 3.3 also sets CMOS clock)
        (DESQview also accepts CX = 4445h and DX = 5351h, i.e. 'DESQ' as valid)
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