https://github.com/PaulStoffregen/Time/

time.c - low level time and date functions
Copyright (c) Michael Margolis 2009-2014
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA

1.0  6  Jan 2010 - initial release
1.1  12 Feb 2010 - fixed leap year calculation error
1.2  1  Nov 2010 - fixed setTime bug (thanks to Korman for this)
1.3  24 Mar 2012 - many edits by Paul Stoffregen: fixed timeStatus() to update
				 status, updated examples for Arduino 1.0, fixed ARM
				 compatibility issues, added TimeArduinoDue and TimeTeensy3
				 examples, add error checking and messages to RTC examples,
				 add examples to DS1307RTC library.
1.4  5  Sep 2014 - compatibility with Arduino 1.5.7
