CURLOPT_TIMECONDITION - Man Page

#include <curl/curl.h>

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 has 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_TIMECOND_NONE (0)

This functionality affects http only

int main(void)
{
  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,
                     (long)CURL_TIMECOND_IFMODSINCE);

    /* Perform the request */
    curl_easy_perform(curl);
  }
}

Added in curl 7.1

Returns CURLE_OK

CURLINFO_FILETIME(3), CURLOPT_TIMEVALUE(3)

curl_easy_setopt(3), curl_getdate(3), CURLINFO_CONDITION_UNMET(3), CURLOPT_RTSP_REQUEST(3), CURLOPT_TIMEVALUE(3), CURLOPT_TIMEVALUE_LARGE(3), libcurl-symbols(3).