It's not that simple: the "difference" between to dates is a timespan - which is not related to any particular day of the year, and so can't be expressed as a "months", "weeks" and "days" count - the numbers you would get would vary considerably if the start date was February 27th to those you would get for March 27th for example.
What you want is a timespan that is relative to a particular date - an Age:
Working with Age: it's not the same as a TimeSpan![
^]