When I hover over the activity items in the Course completion report in 3.10, I do see the activity date and time and not the general course completion time on each activity, for users still working and for users already completed. When a user completes the last activity, it doesn't change the earlier activity completion times. Those times are also correct in the download file. That's how I expect it to work.
Are you referring to the individual user Completion progress details report which only shows the date? Or some other report?