If you're really going to try seriously to answer that question, you're going to need to decide what it means before you get too far along. For example:
How many months are there between December 30, 2016 and January 2, 2017? That could plausibly be zero, one, or two depending on what rules you decide to apply. You can easily think up similar
test cases for weeks and years, but they would all likely have the same sort of answer depending on what you do with partial weeks, months, and years.