Here is a link to a video that details the entire process
https://www.youtube.com/watch?v=GBYWKdoVrOM
And here are the code snippets that you'll want to use
'Beginning of Current Month (EOM)
DateSerial(Year(Date.Now), Month(Date.Now), 1)
'Beginning of Last Month (BOM)
DateAdd(DateInterval.Month, -1, DateSerial(Year(Date.Now), Month(Date.Now), 1))
'End of Last Month (EOM)
DateAdd(DateInterval.Minute, -1, DateSerial(Year(Date.Now), Month(Date.Now), 1))