When I participated in the study that led to "A Wandering Mind Is an Unhappy Mind" I answered questions on my phone that insisted that I answer as soon as I got the notification. This made me assume that there is a significant change in emotional perception as time passes, however I can't find any evidence of this. In other words, how is the experience of an emotion affected by the amount of time between it's experience and it's recollection? If this question is too broad, it can be narrowed down to pain and pleasure.
As shown in the Wikipedia section "Memory of felt emotion" there are many ways that recollected emotion is significantly different than experienced emotion. From the section:
Another study found that people's memories for how distressed they felt when they learned of the 9/11 terrorist attacks changed over time and moreover, were predicted by their current appraisals of the impact of the attacks (Levine et al., 2004). It appears that memories of past emotional responses are not always accurate, and can even be partially reconstructed based on their current appraisal of events. 
Studies have shown that as episodic memory becomes less accessible over time, the reliance on semantic memory to remember past emotions increases. In one study Levine (2009) primes of the cultural belief of women being more emotional than men had a greater effect on responses for older memories compared to new memories. The long term recall of emotions was more in line with the primed opinions, showing that long term recall of emotions was heavily influenced by current opinions.
 Levine L.J. and Pizarro D.A. (2004) "Emotion and memory research: A grumpy overview" Social Cognition, Vol. 22, No. 5, 2004, pp.530-554
 Levine; Lench, Heather; Safer, Martin (2009). "Functions of Remembering and Misremembering Emotion". Applied Cognitive Psychology. 23: 1059–1075.