text: // Code by JeeLabs http://news.jeelabs.org/code/
  // Released to the public domain! Enjoy!
  
  #ifndef _RTCLIB_H_
  #define _RTCLIB_H_
  
  #include <Arduino.h>
  class TimeSpan;
  
  
  #define PCF8523_ADDRESS       0x68
  #define PCF8523_CLKOUTCONTROL 0x0F
  #define PCF8523_CONTROL_3     0x02
  
  #define DS1307_ADDRESS  0x68
  #define DS1307_CONTROL  0x07
  #define DS1307_NVRAM    0x08
  
  #define DS3231_ADDRESS  0x68
  #define DS3231_CONTROL  0x0E
  #define DS3231_STATUSREG 0x0F
  
  #define SECONDS_PER_DAY 86400L
  
  #define SECONDS_FROM_1970_TO_2000 946684800
  
  
  
  // Simple general-purpose date/time class (no TZ / DST / leap second handling!)
  class DateTime {
  public:
      DateTime (uint32_t t = 0);
      DateTime (uint16_t year, uint8_t month, uint8_t day,
                  uint8_t hour = 0, uint8_t min = 0, uint8_t sec = 0);
      DateTime (const DateTime& copy);
      DateTime (const char* date, const char* time);
      DateTime (const __FlashStringHelper* date, const __FlashStringHelper* time);
      uint16_t year() const       { return 2000 + yOff; }
      uint8_t month() const       { return m; }
      uint8_t day() const         { return d; }
      uint8_t hour() const        { return hh; }
      uint8_t minute() const      { return mm; }
      uint8_t second() const      { return ss; }
      uint8_t dayOfTheWeek() const;
  
      // 32-bit times as seconds since 1/1/2000
      long secondstime() const;   
      // 32-bit times as seconds since 1/1/1970
      uint32_t unixtime(void) const;
  
      DateTime operator+(const TimeSpan& span);
      DateTime operator-(const TimeSpan& span);
      TimeSpan operator-(const DateTime& right);
  
  protected:
      uint8_t yOff, m, d, hh, mm, ss;
  };
  
  // Timespan which can represent changes in time with seconds accuracy.
  class TimeSpan {
  public:
      TimeSpan (int32_t seconds = 0);
      TimeSpan (int16_t days, int8_t hours, int8_t minutes, int8_t seconds);
      TimeSpan (const TimeSpan& copy);
      int16_t days() const         { return _seconds / 86400L; }
      int8_t  hours() const        { return _seconds / 3600 % 24; }
      int8_t  minutes() const      { return _seconds / 60 % 60; }
      int8_t  seconds() const      { return _seconds % 60; }
      int32_t totalseconds() const { return _seconds; }
  
      TimeSpan operator+(const TimeSpan& right);
      TimeSpan operator-(const TimeSpan& right);
  
  protected:
      int32_t _seconds;
  };
  
  // RTC based on the DS1307 chip connected via I2C and the Wire library
  enum Ds1307SqwPinMode { OFF = 0x00, ON = 0x80, SquareWave1HZ = 0x10, SquareWave4kHz = 0x11, SquareWave8kHz = 0x12, SquareWave32kHz = 0x13 };
  
  class RTC_DS1307 {
  public:
      bool begin(void);
      static void adjust(const DateTime& dt);
      uint8_t isrunning(void);
      static DateTime now();
      static Ds1307SqwPinMode readSqwPinMode();
      static void writeSqwPinMode(Ds1307SqwPinMode mode);
      uint8_t readnvram(uint8_t address);
      void readnvram(uint8_t* buf, uint8_t size, uint8_t address);
      void writenvram(uint8_t address, uint8_t data);
      void writenvram(uint8_t address, uint8_t* buf, uint8_t size);
  };
  
  // RTC based on the DS3231 chip connected via I2C and the Wire library
  enum Ds3231SqwPinMode { DS3231_OFF = 0x01, DS3231_SquareWave1Hz = 0x00, DS3231_SquareWave1kHz = 0x08, DS3231_SquareWave4kHz = 0x10, DS3231_SquareWave8kHz = 0x18 };
  
  class RTC_DS3231 {
  public:
      bool begin(void);
      bool isrunning(void);
      static void adjust(const DateTime& dt);
      bool lostPower(void);
      static DateTime now();
      static Ds3231SqwPinMode readSqwPinMode();
      static void writeSqwPinMode(Ds3231SqwPinMode mode);
  private:
      bool is_initialized_ = false;
  };
  
  
  // RTC based on the PCF8523 chip connected via I2C and the Wire library
  enum Pcf8523SqwPinMode { PCF8523_OFF = 7, PCF8523_SquareWave1HZ = 6, PCF8523_SquareWave32HZ = 5, PCF8523_SquareWave1kHz = 4, PCF8523_SquareWave4kHz = 3, PCF8523_SquareWave8kHz = 2, PCF8523_SquareWave16kHz = 1, PCF8523_SquareWave32kHz = 0 };
  
  class RTC_PCF8523 {
  public:
      bool begin(void);
      void adjust(const DateTime& dt);
      bool initialized(void);
      static DateTime now();
  
      Pcf8523SqwPinMode readSqwPinMode();
      void writeSqwPinMode(Pcf8523SqwPinMode mode);
  };
  
  // RTC using the internal millis() clock, has to be initialized before use
  // NOTE: this clock won't be correct once the millis() timer rolls over (>49d?)
  class RTC_Millis {
  public:
      static void begin(const DateTime& dt) { adjust(dt); }
      static void adjust(const DateTime& dt);
      static DateTime now();
  
  protected:
      static long offset;
  };
  
  #endif // _RTCLIB_H_
  