Int 1Ah, 03h            Set Real-Time Clock Time                         many

    Sets the time for the computer's real-time clock.

        Entry   AH = 03h
                CH = Hours (BCD)
                CL = Minutes (BCD)
                DH = Seconds (BCD)
                DL = 1 if daylight saving time option; else 0

        Return  Nothing

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    All times are in Binary Coded Decimal (BCD). For example, 9:04.12
    am would be set as CX = 0904; DH = 12.

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

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