I too need this feature and won't upgrade to a pro version unless this can be made to work. For instance, I need project tracking for my people and other than this one feature everything seems to fit our needs. However, not being able to enter the actual completion date makes the data useless.
From a management point of view, one of the suggestions of having a checkoff date and a completion date would be very useful as well.
Also, it appears that lacking this feature makes the goal chain tool useless as well, as not getting the task checked on the correct day seems to break the chain. Perhaps there is a way to fix this behavior?
Anyway for us, the value of the pro account is in the data, history, progress, tracking etc. So without the ability to enter correct completion dates (times to, as a matter of fact), the tool is of limited utility.
One comment was that third party apps could be used to add this feature. Clunky, but perhaps workable. Can anyone explain how that works?