Never wake up too late - Alarm Clock with annoying tone

/* Alarm Clock
  by smi1100 - 06/06/2019
  My first project. The following components are needed:
  - Arduino Uno
  - DS3231 real time clock
  - 20x4 I2C Display
  - buzzer
  - 5 buttons
  - 5 resistors 10k Ohm
  - bread board
  - casing


/* modul 0 - I2C Bus
  - I2C adresses
    - LCD 0x27
    - real time clock
      RTC DS3231 0x68
      EEPROM AT24C32 0x57 (not in use)
*/

#include <Wire.h>


/* modul 1 - DS3231 real time clock
  - connections (from right side) - GND, VCC, SDA, SCL
  - RTClib.h is required
  - I2C connection
*/

#include "RTClib.h"
RTC_DS1307 rtc;
char weekday[7][12] = {"Sonntag", "Montag", "Dienstag", "Mittwoch", "Donnerst.", "Freitag", "Samstag"};
// in english char weekday[7][12] = {"Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday"};


/* modul 2 - Joy-IT SBC-LCD 20x4 display
  - 4 rows, 20 characters
  - connections (from right side) - GND, VCC, SDA, SCL
  - LiquidCrystal_I2C.h is required
  - I2C connection
*/

#include <LiquidCrystal_I2C.h>
LiquidCrystal_I2C lcd(0x27, 20, 4); // adress 0x27
//LiquidCrystal_I2C lcd(0x3F,20,4); // if the lcd is not working, change adress to 0x3F


/* modul 3 - buttons to adjust the alarm time and button for the buzzer
*/

int buzzer = 4;
int buttonhourup = A3;
int buttonhourdown = A2;
int buttonminuteup = A1;
int buttonminutedown = A0;
int readbuttonhourup;
int readbuttonhourdown;
int readbuttonminuteup;
int readbuttonminutedown;

// alarmtime
int alarmtimehour;
int alarmtimeminute;


/* modul 4 - switch for alarm on/off
  - OneButton.h is required
  - see internet resource https://www.makerblog.at/2015/01/arduino-und-pushbuttons-drucktaster-einbinden-mit-der-onebutton-library/
*/

#include "OneButton.h"
int buttonalarm = 5;
OneButton button(buttonalarm, false);
int alarmstate = 1;             // equal to alarm is on 
int alarmtimeequaltime = 0;     // active when alarm time is equal to time


/* modul 5 - EEPROM - backup of alarmtime in the EEPROM
  - EEPROM.h is required
*/

#include "EEPROM.h"


/* code 1 - summer or winter time
  - see internet resource https://kaufma.net/blog/article/2016/08/24/sommerzeit-im-microcontroller/
  - see internet resource https://electronicfreakblog.wordpress.com/2014/03/06/die-zeit-im-sommer-und-im-winter/
  DST = Daylight Saving Time, equal to summer time
*/

int DST;


/* code 2 - changing buzzer sound
*/

unsigned long previousMillis = 0;
const long interval = 1000;


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

  // read alarmtime from EEPROM

  alarmtimehour = EEPROM.read(0);
  alarmtimeminute = EEPROM.read(1);


  // initialize DS3231 RTC

  Serial.begin(9600);
  while (!Serial); // for Leonardo/Micro/Zero

  if (! rtc.begin())
  {
    Serial.println("Couldn't find RTC");
    while (1);
  }

  if (! rtc.isrunning())
  {
    // define format once
    Serial.println("RTC is NOT running!");
    rtc.adjust(DateTime(F(__DATE__), F(__TIME__)));
    Serial.println("RTC adjusted!");
  }


  // initialize display

  lcd.init();
  lcd.backlight();


  // initialize buttons and buzzer

  pinMode(buttonhourup, INPUT);
  pinMode(buttonhourdown, INPUT);
  pinMode(buttonminuteup, INPUT);
  pinMode(buttonminutedown, INPUT);
  pinMode(buzzer, OUTPUT);


  // initialize button as a switch

  pinMode(buttonalarm, INPUT);
  button.attachClick(clickedIt);
 
}

void loop ()
{

//determine summer or winter time

int DST;

DateTime now = rtc.now();

if (now.month() == 3  && now.day() > 24 && now.dayOfTheWeek() == 1 && now.hour() >= 2 || //exakter Tag Beginn
    now.month() == 3  && now.day() + 8 - now.dayOfTheWeek() > 31 && now.dayOfTheWeek() != 1 || // restlichen Tage des Mrz
    now.month() >  3 && now.month() < 10 || //April bis September
    now.month() == 10 && now.day() + 8 - now.dayOfTheWeek() < 32 || //Oktober bis Tag vor letztem Sonntag
    now.month() == 10 && now.day() > 24 && now.dayOfTheWeek() == 1 && now.hour() <= 3) //letzter Sonntag
    {
    DST = 1;
    Serial.print("summertime - ");
    Serial.print("time: ");
    Serial.println(now.hour()+DST);
    }
else 
    {
    DST = 0;
    Serial.print("wintertime - ");
    Serial.print("time: ");
    Serial.println(now.hour());
   }


// button as a switch

  button.tick();
  delay(10);

// shown data on the LCD

  lcd.setCursor(1, 0);

  lcd.print(weekday[now.dayOfTheWeek()]);
  lcd.print("   ");

  lcd.setCursor(13, 0);

  if (now.day() < 10) lcd.print("0");            // less than 10 -> add 0 before it
  lcd.print(now.day(), DEC);
  lcd.print(".");

  if (now.month() < 10) lcd.print("0");          // less than 10 -> add 0 before it
  lcd.print(now.month(), DEC);
  //lcd.print(".");
  //lcd.print(now.year(), DEC);


  lcd.setCursor(0, 1);
  lcd.print("   Zeit      Alarm");
  // in english lcd.print("   time      alarm");

  lcd.setCursor(0, 2);

  lcd.print(" ");
  if ((now.hour() + DST) < 10) lcd.print("0");       // less than 10 -> add 0 before it
  lcd.print((now.hour() + DST), DEC);
  lcd.print(":");

  if (now.minute() < 10) lcd.print("0");       // less than 10 -> add 0 before it
  lcd.print(now.minute(), DEC);
  lcd.print(":");

  if (now.second() < 10) lcd.print("0");       // less than 10 -> add 0 before it
  lcd.print(now.second(), DEC);
  lcd.print("    ");

  if (alarmtimehour < 10) lcd.print("0");       // less than 10 -> add 0 before it
  lcd.print(alarmtimehour, DEC);
  lcd.print(':');

  if (alarmtimeminute < 10) lcd.print("0");       // less than 10 -> add 0 before it
  lcd.print(alarmtimeminute, DEC);

  lcd.setCursor(14, 3);
  if (alarmstate == 1)lcd.print("EIN");
  else lcd.print("AUS");
  /* in english if (alarmstate == 1)lcd.print("ON");
  else lcd.print("OFF");
  */

  // adjust the alarm time (hour)

  readbuttonhourup = digitalRead(buttonhourup);

  if (readbuttonhourup == HIGH)
  {
    delay(100);
    if (alarmtimehour >= 23) alarmtimehour = 0;
    else alarmtimehour ++;
  }


  readbuttonhourdown = digitalRead(buttonhourdown);

  if (readbuttonhourdown == HIGH)
  {
    delay(100);
    if (alarmtimehour <= 0) alarmtimehour = 23;
    else alarmtimehour --;
  }


  // adjust the alarm time (minute)

  readbuttonminuteup = digitalRead(buttonminuteup);
  if (readbuttonminuteup == HIGH)
  {
    delay(100);
    if (alarmtimeminute >= 59) alarmtimeminute = 0;
    else alarmtimeminute ++;
  }


  readbuttonminutedown = digitalRead(buttonminutedown);
  if (readbuttonminutedown == HIGH)
  {
    delay(100);
    if (alarmtimeminute <= 0) alarmtimeminute = 59;
    else alarmtimeminute --;
  }

  // write alarmtime to EEPROM

  EEPROM.write(0, alarmtimehour); // first character = position, second character = value
  EEPROM.write(1, alarmtimeminute);


  // check alarm time

  if ((now.hour() + DST) == alarmtimehour && now.minute() == alarmtimeminute && now.second() == 0 && alarmstate == 1)
  {
    alarmtimeequaltime = 1;
  }

  // activate alarm

  if (alarmtimeequaltime == 1 && alarmstate == 1)
  // it is not possible to use the command "delay" - the display of the seconds will stop
  {
    unsigned long currentMillis = millis();

    if (currentMillis - previousMillis >= interval)
    {
      tone(buzzer, 1000);
      if (currentMillis - previousMillis >= 2*interval)
      {
      previousMillis = currentMillis;
      }
    }
  
    else
    {  
    tone(buzzer, 300);
    }
  }
  
  if (alarmtimeequaltime == 1 && alarmstate == 0)
  {
    noTone(buzzer);
    alarmtimeequaltime = 0;
  }
    
}


void clickedIt()
{
  alarmstate = !alarmstate;
}