Dear Yannick,
Your calculation looks nearly perfect, the only issue is that your way of calling the "test_date" variable is not 100% correct.
In a longitunal study design, it's good practice to refer to the event name (and not the form name!) before calling the variable.
As such, since your "Patient information" instrument (which contains "test_date") is assign to the "Baseline" event (unique event name: baseline_arm_1), your calculation should look like:
@CALCDATE([baseline_arm_1][test_date],1,'M')
Best,
C.