The "difference in years" could mean a number of
different things, for example:

    - If T1 is 31-Dec-2007 23:59:59 and T2 is 01-Jan-2008
      00:00:01, are T1 and T2 a year apart (they occur in
      different calendar years), or about 0.0000000634 years
      apart (two seconds)?

    - Same situation, but assuming that the chronology wizards
      add a leap second 23:59:60 to the end of December 2007.

    - Same situation, but assuming that T1 is given in EDT
      (UTC-5) and T2 in UTC.

    - If T1 is 01-Jan-2008 00:00:00 UTC and T2 is 01-Jan-2009
      00:00:00 UTC, are they one year apart, or a year and a
      day apart, or a year and 18 hours (approximately) apart?

    - If T1 is the birthday of Iannis Xenakis (29-May-1922) and
      T2 is the date of his death (04-Feb-2001), how many years
      apart are T1 and T2?  Keep in mind that Xenakis was a Greek
      composer, so the dates are given according to the calendars
      used in Greece at the relevant times.  Greece switched from
      the Julian to the Gregorian calendar in 1923, so in that year
      15-Feb was followed by 01-Mar; 1923 was only 352 days long in
      Greece, about 3.56% shorter than most other years.  Does that
      affect your answer?

    In short, before you can get a useful answer you need to
clarify your question.  Exactly what do you mean by "the difference
in years" between two moments?

Use your Yankee engineering and the getTime() method of the Date class
if you know how many milliseconds there are between two dates, you can
cipher it out...
Assuming the two dates are date1 and date2 you can try this...
long dateDiffms = date1.getTime() - date2.getTime()
double yrsDiff = dateDiffms / (1000 * 60 * 60 * 24 * 365);   // assuming its
not a leap year
now you can use the Math class to round yrsDiff or use Math.floor if you say
you're not 21 until after your 21st birthday.
a lot of the Date's methods are deprecated, so you may want to use the
GregorianCalendar class.