title: MySQL date difference
body: <p>I am using MySQL and PHP, and I want to find the difference between two dates.</p>
  
  <p>I have a table named <code>advertisers</code>, which has a field <code>web_start_date</code>. I want to select all entries where the <code>web_start_date</code> is less than 30 days from the current date</p>