This bug has existed since version 1.0, was reported many times and never been fixed. The "time to complete" estimate that is listed on the colony Build Queue is often incorrect (underestimated by one week). If you compare the Build Queue value to the value listed for that colony on the Planet/Ship List (list that sits on top of the main map on the left side of the screen) you will find that the Planet/Ship List is always correct.
Please fix this irritating bug.