Log current time when finishing task

Currently there is a log 'Finished executing task xxx', it will be better if it could also log the current time like 'Finished executing task xxx at 2017-09-25 11:01:30', or log as part of task summary.

The reason I want this is because I often forget if the build is the latest or the one I build before I made some changes.

Start by locating the existing "Finished executing task" log message and the task-completion path that emits it. Check whether the timestamp belongs in that message or the task summary, then verify that completed-task output includes the current time in the requested format.

Written by the indexing model from the issue text.