RasClock is the only one support by FFP. i.e. they include the driver. I assume it will work with other RTC but you would need to muck around. RasClock V4 US$18 total to OZ.
One thing I saw on the Falcon website is that you really only want to use the rasclock if you aren't connected to a network because the FPP only reads from the rasclock at powerup whereas the NTP service will be continually correcting your time. If you use a rasclock but also have it connected to the internet you will want to keep the NTP capability enabled.