Welcome to the forum @mizer666
Every AT command for the RAK3172 should have a corresponding API call. It might not yet be in the documentation, as the time request is a quite new feature.
I found an example in the header files, but I am not sure if this is sufficient.
As far as I know you will need at least one uplink to the LNS to receive the time and your LNS must support it.
/**@par Description
* This api(a) allows the user to get the local time in a UTC format
*
* @par Syntax
* api.lorawan.ltime.get()
*
* @return the local time in a UTC format (Type: bool)
* @par Example
* @verbatim
// OTAA Device EUI MSB
uint8_t node_device_eui[8] = {0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08};
// OTAA Application EUI MSB
uint8_t node_app_eui[8] = {0x0E, 0x0D, 0x0D, 0x01, 0x0E, 0x01, 0x02, 0x03};
// OTAA Application Key MSB
uint8_t node_app_key[16] = {0x2B, 0x7E, 0x15, 0x16, 0x28, 0xAE, 0xD2, 0xA6, 0xAB, 0xF7, 0x15, 0x88, 0x09, 0xCF, 0x4F, 0x3C};
void setup()
{
Serial.begin(115200);
api.lorawan.appeui.set(node_app_eui, 8);
api.lorawan.appkey.set(node_app_key, 16);
api.lorawan.deui.set(node_device_eui, 8);
api.lorawan.band.set(4);
api.lorawan.njm.set(1);
api.lorawan.deviceClass.set(1);
api.lorawan.join();
//wait for Join success
while (api.lorawan.njs.get() == 0)
{
Serial.print("Waiting for Lorawan join...");
api.lorawan.join();
delay(10000);
}
}
void loop()
{
Serial.printf("The local time(UTC) is %s\r\n", api.lorawan.ltime.get().c_str());
delay(1000);
}
@endverbatim
*/