Getting both is only $5 more, and provides not only the ability to use it where there's no WiFi/Internet, but also provides a temperature sensor. Plus, the RTC isn't offered separately, so might as well get both to start, just for the flexibility.
Yep that makes sense. And given what I found in the code it should "just work".
Just FYI - you actually could buy the RTC separately if you wanted to. It's not listed on the store site but Ian does sell them if you contact him. Not sure about price tho. Not suggesting you do that, just leaving this here for the next person who comes across this thread.
Look into it later when the dust is clearing off the crater.