As far as just a basic traffic monitor for interfaces on pfSense? This is probably the best. Others do a lot more, like tracking individual hosts/devices, what they're connecting to, etc. and might not track the interface as a whole.
To be honest, as long as you know the time difference, it's still doing what it should be doing. Plus, while my hourly graph seems to be horribly screwed up (it's showing that the current hour is 10/22/2020 11:00 AM), the daily and monthly graphs seem to be fine. And it's that monthly graph that you'd be most concerned about when checking against a data cap.