I'm a blistering idiot. The [Stop Arrival DateTime] and the [Stop Month] field reside on different tables. This leads to their connecting key being very important, but also very hidden since I thought they were on the same table. This was my final solution.
=If([StopCalendarDate] > Date(Max(Total [StopCalendarDate]) - 120)
AND [StopCalendarDate] <= Date(Max(Total [StopCalendarDate])), StopMonth)