I have seen many questions and answers on this topic, but none addressed my particular problem. I extended the java Calendar class (standard--no third party libraries), and needed to find the difference in days between two arbitrary dates.
As suggested by other users, you need to also set the millisecond value of the calendars to zero to compare only the dates. This can be achieved with the following code snippet:
Also, note that timezone changes (e.g. moving from winter time to summer time) may mean there is more or less than 86,400,000 ms in a day.