CURLcode curl_easy_setopt(CURL *handle, CURLOPT_TIMECONDITION, long cond);
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.
CURL *curl = curl_easy_init(); if(curl) { curl_easy_setopt(curl, CURLOPT_URL, "https://example.com"); /* January 1, 2020 is 1577833200 */ curl_easy_setopt(curl, CURLOPT_TIMEVALUE, 1577833200L); /* If-Modified-Since the above time stamp */ curl_easy_setopt(curl, CURLOPT_TIMECONDITION, CURL_TIMECOND_IFMODSINCE); /* Perform the request */ curl_easy_perform(curl); }