title: Subtracting Dates in Unix
body: <p>is there a way to subtract a month from a date or at least 30 days in unix.</p>
  
  <p>example:</p>
  
  <p>[yyy-mm-dd] - 30 days</p>
  
  <p>2011-07-23 - 30 days</p>
  
  <p>By the way, the date can be any date depending on the user input.</p>
  
  <p>Thanks.</p>