CURLOPT_TIMECONDITION − select condition for a time request
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TIMECONDITION, long cond);
Pass a long as parameter. This defines how the CURLOPT_TIMEVALUE(3) time value is treated. You can set this parameter to CURL_TIMECOND_IFMODSINCE or CURL_TIMECOND_IFUNMODSINCE.
The last modification time of a file is not always known and in such instances this feature will have no effect even if the given time condition would not have been met. curl_easy_getinfo(3) with the CURLINFO_CONDITION_UNMET option can be used after a transfer to learn if a zero-byte successful "transfer" was due to this condition not matching.
HTTP, FTP, RTSP, and FILE