Jahr in Datum ersetzen (SUBDT)

[Fuerchau]

%YEARS converts a number into a duration that can be added to a date or
timestamp value.
%YEARS can only be the right-hand value in an addition or subtraction operation.
The left-hand value must be a date or timestamp. The result is a date or timestamp
value with the appropriate number of years added or subtracted. For a date, the
resulting value is in *ISO Format.

Hier hast du die Erklärung.
Du kannst noch minimal abkürzen:
DatumS += %Years(%SubDt(DatumT: *Y) - %SubDt(DatumS: *Y));