You may consider the following concept. Philips has an RTC chip that comes in small 8 pin package equipped with the SCL & SDA lines for an I2C interface to the 8051. This part, typical of most RTCs has the normal counters down to SECONDS. However the part has an onboard clock divider that divides the 32.768 KHz crystal oscillator of the RTC down to a lower rate and then makes this output available as a pin to the 8051. This chip, called the PCF8562 (data sheet at philips web site) can have this output generate pulses at 1024 Hz. This is a little faster than the 1 Msec period you were looking for,.....but no matter, you can easily connect this to either INT0 or INT1 of the processor and get a real time periodic interrupt to millisecond level time passage.

Most RTCs these days have serial interfaces that take a certain amount of time to get the data into and out of the chip. For this reason it is not too useful to have an onboard register available for access to a milliseconds counter. (By the time it has been read out it is likely that the count has already changed on the part.) Another reason is that such a register, if part of the RTC structure itself, would increase the current draw for the RTC chip thus shortening battery life.

It also turns out that it is not directly possible to divide the 32.768 KHz frequency directly down to exactly 1 KHz (1 msec). For this reason some RTC chips have an option for operation with a 32.000 KHz crystal.

Hope this helps
Michael Karas