Calculates the number of years between two dates.
If the first date is before the second date, the returned number is positive. If
the first date is after the second date, the returned number will be negative.
years between(date{4 April 1999}, date{4 April 2000})→1{year}
years between(date{4 April 1999}, date{5 April 2000})→1{year} + (1{year}/365)
years between(date{4 February 1999}, date{5 February 2000})→1{year} + (1{year}/366)