Issues with $START_PERIODKEY[YYYY]$: giving wrong year

Summary:

I've implemented an integration, where I'm trying to leverage $START_PERIODKEY[YYYY]$ unfortunately for Dec-20 it gives me year 2021 but I would have expected 2020

Content (required):

I've implemented an integration, where I'm trying to leverage $START_PERIODKEY[YYYY]$ as Option for a Year paramenter. Unfortunately, for Dec-20 it gives me year 2021 but I would have expected 2020. It works correctly though for Nov-20. Any hints or tips on how to correct this?

Version (include the version you are using, if applicable):

21.09.61

Code Snippet (add any code snippets that support your topic, if applicable):