How to set a creation/completion date for User stories and Tasks different that the system date ?

Hi,
I'm testing Version One "team edition" so far : I'm importing the user stories, 
some of them are affected to a sprint that has already begun.
I don't find any way where I can set a creation date or a completion date for 
given task or user story different that the date where you enter the data, so 
it's impossible to get a proper Burndown chart.
Can you tell how to achieve that ?
Thanks in advance
Marie

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