I read about this in Modern Magazine (yay free copy from Merchandise Mart):
Ink Calendar by Oscar Diaz
Every month starts out as an all-white paper conjoinment of numbers. Place the end in its ink bottle (different colors for different months based on color and perceived climate temperatures) and the paper absorbs the ink so that every day, the new date fills with color.
Wow. Consider my mind blown.
Check out Oscar Diaz's website, including the Ink Calendar (under "Projects").