How does the group that is not stressed and not reactivated act as a control for time in Schwabe & Wolf 2010? Schwabe and Wolf (2010) 


  Rodent studies suggest that memory reconsolidating is impaired by stress. Here we examined in healthy humans the effect of stress on the reconsolidation of autobiographical memories. Participants recalled positive, negative and neutral episodes from their recent past and afterward exposed to a stressor (socially evaluated cold pressor test) or a non-arousing control condition. Additional groups of participants were exposed to the stressor without prior memory reactivation or were neither stressed nor asked to recall episodes from their past. Stress after memory reactivation impaired the memory for the neutral episodes 1 week later....


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? 

References:


Schwabe, L., &amp; Wolf, O. T. (2010). Learning under stress impairs memory formation. Neurobiology of learning and memory, 93(2), 183-188. https://www.psy.uni-hamburg.de/arbeitsbereiche/kognitionspsychologie/publications/schwabe2010b-nlm.pdf
It appears that they tested all of the participants (control &amp; stress) at the same time:


  
  Memory was assessed 24 h after learning. According to the model
  by Joels and colleagues (2006), it can be predicted that learning under
  stress enhances memory, in particular for stress context-related
  information
  Twenty-four hours after learning, participants returned to the
  laboratory and completed a free recall test
  Immediately after the free recall test, participants were given a
  recognition test.
  All testing took place in the morning between 8 am and 1 pm to
  control for the diurnal variation of the stress hormone cortisol. On
  day 1, participants heard a list of eight neutral words that were related
  to the stressor as well as eight negative, eight positive and eight
  neutral, stressor-unrelated words while they were exposed to stress
  or a control condition (for details see below). On the following day,
  participants returned to the laboratory and completed a free recall
  and a recognition test for the words they heard the day before.
  


I can't find anywhere within the text that says "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."

They found that: 


  Learning under stress reduced both free recall and recognition
  performance 24 h after learning.


All taken from your reference:


Schwabe, L., &amp; Wolf, O. T. (2010). Learning under stress impairs memory formation. Neurobiology of learning and memory, 93(2), 183-188.