CURLcode curl_easy_getinfo(CURL *handle, CURLINFO_RETRY_AFTER, curl_off_t *retry);
While the HTTP header might contain a fixed date string, the CURLINFO_RETRY_AFTER(3) will always return number of seconds to wait - or zero if there was no header or the header couldn't be parsed.
CURL *curl = curl_easy_init();
if(curl) {
CURLcode res;
curl_easy_setopt(curl, CURLOPT_URL, "https://example.com");
res = curl_easy_perform(curl);
if(res == CURLE_OK) {
curl_off_t wait = 0;
curl_easy_getinfo(curl, CURLINFO_RETRY_AFTER, &wait);
if(wait)
printf("Wait for %" CURL_FORMAT_CURL_OFF_T " seconds\n", wait);
}
curl_easy_cleanup(curl);
}