TIME: RTC Module

Resources

Adafruit Learning System Adafruit DS3231 Precision RTC Breakout

The DS3231 keeps time even when Arduino is unplugged — it has its own crystal oscillator and backup coin cell battery.

Test Arduino Code

#include <RTClib.h>
#include <Wire.h>

DS3231 rtc;
char buf[20];

void setup() {
  Serial.begin(9600);
  while (!Serial);

  Wire.begin();            // Nano 33 IoT uses Wire on SDA/SCL pins
  rtc.begin();

  if (!rtc.isrunning()) {  // this method belongs to the DS3231 lib, recc'd by Arduino
    Serial.println("RTC is NOT running!");
    rtc.adjust(DateTime(__DATE__, __TIME__));
  }
}

void loop() {
  DateTime now = rtc.now();
  Serial.println(now.tostr(buf));   // <-- using tostr(), not toString()

  Serial.print(" since midnight 1970/1/1 = ");
  Serial.print(now.unixtime());
  Serial.print("s = ");
  Serial.print(now.unixtime() / 86400L);
  Serial.println("d");

  DateTime future(now + (7 * 86400L + 30));
  Serial.print(" now + 7d + 30s: ");
  Serial.println(future.tostr(buf)); // <-- tostr()

  DateTime past(now - 30 * 86400L);
  Serial.print(" now - 30d: ");
  Serial.println(past.tostr(buf));   // <-- tostr()

  Serial.println();
  delay(3000);
}