INT 1Ah,  03h (3)        Set Real-Time Clock Time                        many

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

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

       Returns:       None

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       Notes:         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.