title: Advanced Date-Validation with PHP
body: <p>I've got to validate numerous dates with my current project. Unfortunately, these dates can vary wildly. Examples include:</p>
  
  <ol>
  <li>1983-07-10 (After 1970)</li>
  <li>1492-10-11 (Before 1970, year of Unix Timestamps - this eliminates strtotime() on some systems)</li>
  <li>200 B.C. (Really old...)</li>
  </ol>
  
  <p>Dates will not exceed 9999 b.c., nor will they be future (beyond 'today'). What would be the best way to validate that the values submitted are indeed dates, and proper dates at that?</p>
  
  <p><strong>Updates...</strong></p>
  
  <p>All dates must be sortable within their global list. Meaning dates 1 and 3 above must be comparable to one-another, and sorted ASC or DESC.</p>
  
  <p>I'm fully aware of the calendar-changes that have taken place in the past, and the confusion around these changes. My project assumes the user has already performed the proper calibration to find out the date according to our modern-calendar system. I won't be performing this calibration for them.</p>