Int 1Ah, 05h            Set Real-Time Clock Date                         many

    Sets the date on the computer's real-time clock.

        Entry   AH = 05h
                CH = Century (19 or 20) (BCD)
                CL = Year (BCD)
                DH = Month (BCD)
                DL = Day (BCD)

        Return  Nothing

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    All dates are in Binary Coded Decimal (BCD). For example, January
    15, 1987 will be set as CX = 1987; DX = 0115.

    The real-time clock is the clock that runs even when the computer
    is turned off. A CMOS battery is used so that, even while the
    computer is off, the date, time, and alarm time are maintained.

    This service is available only for ATs, XT-286s, and PC
    Convertibles. Previous machines will report unpredictable results
    (including the state of the Carry Flag).

    Setting the real-time clock date will not affect the system date.
    This is because the real-time clock is read once upon system
    startup, updating the system timer. All other references to date
    use the system timer. If either the real-time clock date is
    changed (via this service) or the system timer is changed (via
    Service 01h), the other date is not changed accordingly.

    Even though the system uses the system timer for date information,
    both the real-time clock and the system timer are updated
    continuously.