如何在Prolog中返回年份差值？

:- use_module(library(date)).

% 计算年份差值的谓词
year_difference(Date1, Date2, Difference) :-
    date_time_stamp(Date1, Stamp1),
    date_time_stamp(Date2, Stamp2),
    StampDiff is abs(Stamp1 - Stamp2),
    stamp_date_time(StampDiff, DateTimeDiff, 0),
    DateTimeDiff = date(Y, _, _, _, _, _, _, _, _),
    Difference is Y - 1970.

?- date(Y1, 1, 1, 0, 0, 0, 0, _, _), date(Y2, 1, 1, 0, 0, 0, 0, _, _), year_difference(Y1, Y2, Diff).
Y1 = Y2, Y2 = 1970,
Diff = 0.

?- date(Y1, 1, 1, 0, 0, 0, 0, _, _), date(Y2, 1, 1, 0, 0, 0, 0, _, _), Y2 is Y1 + 5, year_difference(Y1, Y2, Diff).
Y1 = 1970,
Y2 = 1975,
Diff = 5.