I've been asked to rerun an experiment of my choice (exciting!!) (to examine my management abilities in organising an experiment), I've chosen this one out of personal interest. I also however want to improve the experiment in any way I can! 
So far I find that it is quite well though through. 

My question is how is the group that is not stressed and not reactivated (control) act as a control for time? Or rather what does "control for time" mean?
The researchers wrote "To control for the effect of time on memory, we had another control group that was not stressed and did not reactivate experiences from their past." 

I understand that the group makes a good baseline for comparing other results to, but where does time come into it? 

If anything strikes you as particularly questionable/could be improved, please note that as well it would be greatly appreciated. :)

Here is the article http://www.sciencedirect.com/science/article/pii/S1074742710000845