Date stamp on a SD file from a real time clock

Using Arduino Programming Questions
2

you would have to initialize the variables before creating your sd card file. As they are global, they would be used by the call back. You can create a function updateVariablesForTimeStamp() that you would call before creating files.

Assuming you use a DS3231, that's the way to read the 6 values

#include <RTClib.h> // https://github.com/adafruit/RTClib
RTC_DS3231 rtc;

unsigned int year = 2019;
byte month = 11;
byte day = 15;
byte hour = 15;
byte minute = 45;
byte second = 22;

// CALL THIS FUNCTION BEFORE DOING FILE OPERATIONS
void updateVariablesForTimeStamp()
{
  DateTime now = rtc.now();
  // HERE IS THE DATA YOU NEED TO UPDATE BEFORE THE dateTime Callback is called
  year = now.year();
  month = now.month();
  day = now.day();
  second = now.second();
  minute = now.minute();
  hour = now.hour();
}

void setup() {
  Serial.begin(115200);

  if (! rtc.begin()) {
    Serial.println(F("NO RTC"));
    while (1);
  }

  if (rtc.lostPower()) rtc.adjust(DateTime(F(__DATE__), F(__TIME__))); // Need to initialize RTC, we use the last compile time of this sketch
}

void loop() {
}

(typed here, totally untested)