title: How to know if a date is the previous month or earlier?
body: <p>I'm looking for some conditions to start billing my clients. 
  Every time that my client make a contract with me, i initialize a date in the attribute start_billing_at. I'd like to know now, if the start_billing_at attribute has been initialized during the previous month. Hope that my problem is more clear now</p>
  
  <p>Thx for help</p>
  
  <p>EDIT </p>
  
  <p>I'v got to know if my date is between the first and the last day of the previous month</p>