Ray
at first, I don't say that changing 42h to 43h
can correct your problem!
second,

RTC_EN EQU P2.0
RTC_AS EQU P2.1
RTC_RW EQU P2.2
RTC_DS EQU P2.3
RTC_IRQ EQU P3.2
SMT_OUT EQU P3.3
LCD_RS EQU P3.5
LCD_RW EQU P3.6
LCD_EN EQU P3.7
ADRSR EQU 5AH
ADRSW EQU 5CH
DATAR EQU 5DH
DATAW EQU 5EH

if with this change your problem still is existed
please say what are you doing in isr?
i think it is better that change your circuit configuration
excuse me if it is not good idea
but in this situation yau can easily read and write from rtc
see


RTC_READ:
SETB RTC_EN
CLR RTC_RW
CLR RTC_DS
CLR RTC_AS
SETB RTC_DS
SETB RTC_RW
SETB RTC_AS
CLR RTC_EN
MOV P1,ADRSR
CLR RTC_AS
CLR RTC_DS
MOV P1,#0FFH
MOV DATAR,P1
SETB RTC_DS
SETB RTC_EN
SETB RTC_AS
RET


RTC_WRITE:

SETB RTC_EN
CLR RTC_RW
CLR RTC_DS
CLR RTC_AS
SETB RTC_DS
SETB RTC_RW
SETB RTC_AS
CLR RTC_EN
MOV P1,ADRSW
CLR RTC_AS
CLR RTC_RW
MOV P1,DATAW
SETB RTC_RW
SETB RTC_EN
SETB RTC_AS
RET

Regards
Mehdi