Luckily Microchip have done this for you - look in the application notes
at their web site

> ----------
> From:         Bonassina Ivano[SMTP:bonassin@TIN.IT]
> Reply To:     pic microcontroller discussion list
> Sent:         Wednesday, December 17, 1997 4:58 AM
> To:   PICLIST@MITVMA.MIT.EDU
> Subject:      CLOCK-CALENDAR ... PCF8583 ... PIC CODE ?
>
> Someone can help me to find an ASM code for PIC ( 16C84 in better ) to
> realize, like an hobbiest, a simple and normal CLOCK ?
> Thanks in advance.
> Bye Ciao
>