Arduino Alarm/Time Discrepancy

Official Hardware UNO Family UNO R4 WiFi
1

Hi. I am using my Arduino to make an IoT project, and its connection to the internet is good. I programmed it to make an alarm go off at 8:00 and 20:00 (8:00 PM). Despite, it goes off at 7:33 and 19:33 (7:33 PM). If I make it print the time over serial, it is accurate, there is no time drift. If I reset the Arduino occasionally (press the reset button), then it works fine. Sometimes it even goes off at 7:33 and 8:00. Is anyone able to spot my oversight? Thanks for your time.
Here is my main code (thingProperties.h doesn't matter):

#include "thingProperties.h"
#include <DHT.h>

#define DHTPIN 2      // Pin where DHT11 is connected
#define DHTTYPE DHT11 // Sensor type
int BUZZER = 3;      // Buzzer
int BTN = 4;         // Interface button
bool isTime = false;
DHT dht(DHTPIN, DHTTYPE);

float tempAdjust = 5;
float humidAdjust = -5;
bool rang = false;

String MorningAlarmTime = "08:00";
String AfternoonAlarmTime = "20:00";  // Example for 3:00 PM

void checkAlarm() {
    time_t now = ArduinoCloud.getLocalTime();
    struct tm *timeinfo = localtime(&now);

    int currentHour = timeinfo->tm_hour;
    int currentMinute = timeinfo->tm_min;

    int morningHour = MorningAlarmTime.substring(0, 2).toInt();
    int morningMinute = MorningAlarmTime.substring(3, 5).toInt();

    int afternoonHour = AfternoonAlarmTime.substring(0, 2).toInt();
    int afternoonMinute = AfternoonAlarmTime.substring(3, 5).toInt();
  
     if (serialOutput) {
      Serial.print("Time is ");
      Serial.print(currentHour);
      Serial.print(":");
      Serial.println(currentMinute);
    }
  
    bool matchMorning = (
        (currentHour == morningHour || currentHour == morningHour + 12) &&
        currentMinute == morningMinute
    );

    bool matchAfternoon = (
        (currentHour == afternoonHour || currentHour == afternoonHour + 12) &&
        currentMinute == afternoonMinute
    );

    bool match = matchMorning || matchAfternoon;

    if (match && !rang) {
        isTime = true;
    } else if (!match) {
        isTime = false;
        rang = false;
    }
}

void setup() {
  pinMode(BTN, INPUT);
  pinMode(BUZZER, OUTPUT);
  // Initialize serial and wait for port to open
  Serial.begin(115200);
  delay(1500);

  dht.begin(); // Start DHT11 sensor

  // Initialize IoT Cloud properties
  initProperties();

  // Connect to Arduino IoT Cloud
  ArduinoCloud.begin(ArduinoIoTPreferredConnection);
  
  // Debugging level (0 = errors only, 4 = max details)
  setDebugMessageLevel(2);
  ArduinoCloud.printDebugInfo();
}

void loop() {
  ArduinoCloud.update(); // Sync with IoT Cloud
  checkAlarm();
    if (isTime && !rang) {
        while (digitalRead(BTN) == LOW) {  // HIGH means button is is pressed
            if (serialOutput) {
                Serial.println("BEEPING!");
            }
            digitalWrite(BUZZER, HIGH);
            delay(250);
            digitalWrite(BUZZER, LOW);
            delay(250);
            digitalWrite(BUZZER, HIGH);
            delay(250);
            digitalWrite(BUZZER, LOW);
            delay(1000);
        }
        rang = true;  // Only mark it as "rang" after button press
    }

  humidAdjust = humidOffset;
  tempAdjust = tempOffset;
  // Read from DHT11 sensor
  float newTemp = dht.readTemperature();
  float newHum = dht.readHumidity() + humidAdjust;
  
  // if (newHum == humidity) {
  //       humidity += 0.01;  // Temporary change
  // }
  
  if (isnan(newTemp) || isnan(newHum)) {
    if (serialOutput) {
      Serial.println("❌ Failed to read from DHT11 sensor!");
    }
    return;
  }
  float temperatureF = ((newTemp * 9.0 / 5.0) + 32) + tempAdjust;

  // Print values to Serial Monitor
  if(serialOutput) {
    Serial.print("🌡 Temp: ");
    Serial.print(temperatureF);
    Serial.print("°F  |  💧 Humidity: ");
    Serial.print(newHum);
    Serial.println("%");
  }

  // Update IoT Cloud variables
  temperature = temperatureF;
  humidity = newHum;

  delay(5000); // Wait 5 seconds before next read
}
2

You code looks like it will work ok. I suspect the problem lies with ArduinoCloud.getLocalTime(). There are posts which suggest that goes infrequently (by default) to a remote time service and relies on the local clock most of the time. The UNO R4 boards do not have external quartz crystals and rely on the poor stability internal oscillators one of which drives the Renesas internal RTC (ex1 ex2).

Arduino RTC does not stay in sync despite use of ArduinoCloud.getLocalTime(); has one solution as TimeService.setSyncInterval(600). As with any remote service if the service is overloaded by clients making high frequency requests then they are likely to be ignored so I would advise caution on going for a lower value than the cited 600 seconds.