Which datetime PHP format should I use best?

// Upload data to the RadMon.org server
static void connecToRadMonLogger(void)
{
  esp_task_wdt_add(NULL);
 
  WiFiClient wifi;
  HttpClient client = HttpClient(wifi, "radmon.org", 80);

  Serial.println(F("Connection to radmon monitoring platform succeeded!"));

  // Concat data for POST
  // API URL
  String ptr = "/radmon.php?function=submit";
  ptr += "&user=";
  ptr += UserName;
  ptr += "&password=";
  ptr += DataSendingPassWord;
  ptr += "&value=";
  ptr += String(cpm);
  ptr += "&unit=CPM";
  ptr += "&datetime=";
  ptr += String(epoch);

  // Test output
  //Serial.println("Uploaded CPM = " + String(cpm));
  Serial.println("created PTR = " + ptr);

  client.beginRequest();
  client.post(ptr);
  client.sendHeader("Content-Type", "application/x-www-form-urlencoded");
  client.sendHeader("Content-Length", ptr.length());
  client.beginBody();
  client.print("");
  client.endRequest();

  // read the status code and body of the response
  statusCodeRadmon = client.responseStatusCode();

  // Give the client some time to stop
  yield();

  esp_task_wdt_delete(NULL);
 
  Serial.println(F("Connection to radmon monitoring platform Disconnected."));
  Serial.println(F("\n"));
}