The link will take you to where you can get it. There are also some more examples and explanations in the description and comments there. If you have questions, just let me know here or in PM.
The nice thing is that if you go from Feb 1 to Mar 1, it counts as a month rather than 28 or 29 days, and if you go from Mar 1 to Apr 1, it also counts as a month. So it's very accurate without just using the average number of days per month in the calculations. It's the same thing for leap years... Jan 1 2012 to Jan 1 2013 is a year, and so is Jan 1 2011 to Jan 1 2012 even though there is one more day in 2012.
