Deriving a dashboard variable from $__timeStart etc

Hi, I’m struggling to find a way to create a Dashboard variable that picks its value based on the currently selected time range. If it’s less than 30 days ago, I want to provide the string “live”, if it’s older then I want the variable to be “archive”.

I don’t see any way to do this without pretending to query a data source “SELECT x FROM DUAL” style. My panels are using a Flux datasource where I’m currently making this calculation on a per panel basis. But I want to simplify things by pulling this variable back to Dashboard level. So I’m wondering what options I do have to do this fairly simple if / then / else process in some form. Ideas very welcome!