Filling "year" infoobject with result of substring

Question: I am trying to create an update rule using a formula. I am using the
substring formula to extract the year and populate a fiscal year infoobject (type numc). HOWEVER - I keep getting the syntax error that type "string" is wrong for the infoobject. When I simply type '2003' in the formula it works!! However substring(xx,'5','4') where x is of type
char(10) does NOT work. Any ideas??????

Answer:
what is the format of x (or xx ?) DD.MM.YYYY ?
seems (xx,'5','4'), the beginning position not correct ?
another way without formula result = xx+m(n) e.g result = xx+6(4) for above format
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