那请问那个when是时间状语从句吗?整个句子的主干是to imagine a time is difficult

那请问那个when是时间状语从句吗?整个句子的主干是to imagine a time is difficult 如题It is difficult to the point of impossibility for the average under the age of forty to imagine a time when high quality art criticism could be found in most big city newspapers.