An application note (PDF!) on low-power real-time clock from Microchip:
This application note uses the Timer1 module, from a mid-range PIC16CXXX microcontroller, to control a low-power real-time clock. Timer1 was chosen because it has its own crystal which allows the module to operate during sleep. The two events that will wake the device from sleep (for this application) are a keypress and a Timer1 overflow.