DS3231 Real-Time Clock.
- d : Day of the month, 2 digits with leading zeros (01 to 31)
- D : A textual representation of a day, three letters (Mon through Sun)
- j : Day of the month without leading zeros (1 to 31)
- l : A full textual representation of the day of the week (Sunday through Saturday)
- N : ISO-8601 numeric representation of the day of the week (1 for Monday through 7 for Sunday)
- S : English ordinal suffix for the day of the month, 2 characters (st, nd, rd or th. Works well with j)
- w : Numeric representation of the day of the week (0 for Sunday through 6 for Saturday)
- z : The day of the year (0 through 365)
- F : A full textual representation of a month, such as January or March (January through December)
- m : Numeric representation of a month, with leading zeros (01 through 12)
- M : A short textual representation of a month, three letters (Jan through Dec)
- n : Numeric representation of a month, without leading zeros (1 through 12)
- t : Number of days in the given month (28 through 31)
- L : Whether it's a leap year (1 if it is a leap year, 0 otherwise)
- Y : A full numeric representation of a year, 4 digits (Examples: 1999 or 2003)
- y : A two digit representation of a year (Examples: 99 or 03)
- a : Lowercase Ante meridiem and Post meridiem (am or pm)
- A : Uppercase Ante meridiem and Post meridiem (AM or PM)
- g : 12-hour format of an hour without leading zeros (1 through 12)
- G : 24-hour format of an hour without leading zeros (0 through 23)
- h : 12-hour format of an hour with leading zeros (01 through 12)
- H : 24-hour format of an hour with leading zeros (00 through 23)
- i : Minutes with leading zeros (00 to 59)
- s : Seconds, with leading zeros (00 through 59)
- U : Seconds since the Unix Epoch (January 1 1970 00:00:00 GMT)
Tutorials: www.jarzebski.pl/arduino/komponenty/zegar-czasu-rzeczywistego-rtc-ds3231.html
This library uses I2C to communicate, 2 pins are required to interface.
Original Code by JeeLabs http://news.jeelabs.org/code/
First fork by adafruit https://github.com/adafruit/RTClib
