Ken's solution can be fixed for non-leap years by using <= instead of <,
but it breaks down for leap years. e.g.

a := Date year: 2003 month: 3 day: 1.
b := Date year: 2004 month: 2 day: 29.

b year - a year - (a dayOfYear <= b dayOfYear ifTrue: 0 ifFalse: 1) "
outputs 1 "

What seems to work is to let Date handle the leap year case with
#addMonths:. e.g.

yd := b year - a year.
(a addMonths: 12 * yd) > b ifTrue: [ yd - 1 ] ifFalse: [ yd ]. " outputs 0 "
Balázs
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