I don't think most Psoriasis cases can be cleared up that way. Yea, sun does have a very positive effect on psoriasis, but I think it's more of an exception when the conditon clears up completely. And in those cases, it seems to usually come back once the summer is over.
That being said, it still looks as if EC is probably not psoriasis, as I haven't heard of anyone saying that the sun had much of an impact on the severeness of the condition.