On Arduino, you would use Serial.println(buffer) instead of printf(), to display the result on the serial monitor.
Try something like this (you will have to add all required the RTC and/or TimeLib.h stuff):
void setup() {
char buffer[50];
Serial.begin(9600); //correct rate needed here
sprintf(buffer, "%d , %d , %d", now.day(), now.month(), now.year());
Serial.println(buffer);
}
void loop(){}
sprintf() does all sorts of handy things, for example %0d adds leading zeros, which is nice for time and date.