I cant understand whether your question is answered properly (looking both thread didnt find exact answer)
actually when you write =FirstDayOfMonth(Now()) you are calling a function FirstDayOfMonth which is not in your database may you have seen this in some other database and used it in your own databse without copying the function
however you can do it directly without calling a function by using dateserial function
below are some examples
First Day of Month
=DateSerial(Year(Date()),Month(Date()),1)
Last Day of Month
=DateSerial(Year(Date()),Month(Date())+1,0)
First Day of Previous Month
=DateSerial(Year(Date()),Month(Date())-1,1)
Last Day of Previous Month
=DateSerial(Year(Date()),Month(Date()),0)
First Day of Next Month
=DateSerial(Year(Date()),Month(Date())+1,1)
Last Day of Next Month
=DateSerial(Year(Date()),Month(Date())+2,0)
Hope its clear now