With this function you can get the number of years between two
dates. This value is always positive, and incomplete years will be
returned as a fraction.
diff = date_year_span(date_create_datetime(2011, 9, 15, 11, 4, 0), date_current_datetime());
This would set diff to the number of years between 15th September 2011, 11:04.0 and the current date and time.