Prior to assigning a not so trivial task the wide range of time between the earliest and latest completion it would be a good decision for the team to have a meeting and discuss about best possible ways to get the task done even before assigning the completion dates so that a definite way could be found to complete the task. Apart from that a little extra time could be devoted on finding out from records about a similar task previously done and coming up with alternatives to save up on the time that could be given for the task to be completed.