title: Checking for expiring membership
body: <p>In my nodejs server I have 2 types of memberships (Free, Paid). 
  Paid Membership lasts exactly 31 days from the day it's been bought.</p>
  
  <p>When a member buys such a membership I (server-side) <code>UPDATE</code> my database (MySQL) in a field <code>paid_expires</code> which accepts a <code>TIMESTAMP</code> input with the current date plus 31 days.</p>
  
  <p>If my server has X paying members who purchased their memberships on different dates, what is an efficient way of informing everyone 3 hours beforehand that their membership expires?</p>