- CURLINFO_CONDITION_UNMET was added to allow an application to get to know if

the condition in the previous request was unmet. This is typically a time
  condition set with CURLOPT_TIMECONDITION and was previously not possible to
  reliably figure out. From bug report #2565128
  (http://curl.haxx.se/bug/view.cgi?id=2565128)

lib/urldata.h

@@ -1063,6 +1063,8 @@ struct PureInfo {
                     was unretrievable. We cannot have this of type time_t,
                     since time_t is unsigned on several platforms such as
                     OpenVMS. */
+  bool timecond;  /* set to TRUE if the time condition didn't match, which
+                     thus made the document NOT get fetched */
   long header_size;  /* size of read header(s) in bytes */
   long request_size; /* the amount of bytes sent in the request(s) */
   long proxyauthavail; /* what proxy auth types were announced */