INT 1Ah,  05h (5)        Set Real-Time Clock Date                        many

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

       On entry:      AH         05h
                      CH         Century (19 or 20) (BCD)
                      CL         Year (BCD)
                      DH         Month (BCD)
                      DL         Day (BCD)

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       Notes:         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.