I do not know of a uC with a battery backed RTC; however Dallas has battery backed memories (NVRAM) with a built in RTC. The question arose from you giving the impression that you has a '51 derivative with a true RTC, evidently you mean a regular timer (T0, T1, T2) with code to show time.

Knowing this the P89c51Rx2 is a drop in replacement, it adds some functionality you can use or not depending on your application.

PCA is a ProgrammableCounterArray which you can use for PWM, timekeeping or whatever.

Have fun,

Erik