It sounded like a quick change and as the RTC chip does have the date available, it seem like a quick project for a rainy afternoon.
It actually took a lot longer than I thought as I had to also make provision for setting the date, including the year. Then I found I needed to restructure everything and as I made changes, more and more of the code needed to be updated. It's almost been rewritten now.
The new firmware is available on the project page for those that want to try it.
I took the opportunity to make a couple of other changes.
I've removed the need for the crystal on the PIC; the one for the RTC is still needed of course. It wasn't required as the PIC's internal timing is good enough and I needed an additional output pin to connect to my debugger. You can leave the crystal in place if you want and it shouldn't cause any issues. However, you now have two free I/O pins to play with if you need them.
The interrupt called display routine has been completely re-written and is now tiny compared to the original. It causes a lot less flicker and the display runs brighter as a result.