I am using the following formula to get the year:
YEAR(DATEVALUE( ActivatedDate ))
But this ends up whith the following 2.018 instead of 2018.
How do I remove the . from 2.018?
If you're using the number return type, this punctuation occurs automatically, and depends on locale (for example, mine would read "2,018" instead, since I'm in the US). To get it as just text, you need to convert it to text:
TEXT(YEAR(DATEVALUE( ActivatedDate )))
You'll need to change the return type appropriately.