Calculate a sum across years

Content

I have a requirement where I need to determine it has been 20 months since an entity has seen it's first activity (net income <> #missing).  If the threshold hasn't been reached the entity is considered "inactive" for the current year->period in which case a stat account will be set as "inactive" if has been 20 months since first activity then my stat account for it will read "active" for the current year->period .

I have a traditional Planning period and year dimension setup.  My brain is bending trying to come up with a strategy on how to approach this.  Any guidance would be appreciated.