If your system will work round the clock or if it is already battery operated then your job is very simple.

You can spare one counter, if not use 89C52 rather, so you will have one extra timer, write the code to develop a clock, you will need to make the code for setting up time and date etc. as and whenever it is needed.

It is not difficult but lengthy, I developed this code an year ago and it was working for Clock only, it was not meant for date, but this can also be done by using aroound 16 bytes of RAM.
Which you will have 128Byte extra anyway if you use 89C52 instead of 89C51.

Rauf