Now you can have a background task in Trello.

Background task will be tracked if there is no other ongoing tasks only.
Any other (foreground) task will interrupt a background task while in progress and
will restore the background task back after it's done.

What is the background task, how to create it and how it works?
- Any card that has BG label with it (case insensitive) will be counted by 3T as a background task for the current board.
- Any other card on the same board will be counted as foreground task and will interrupt any background task while in progress.
- After a foreground task is done the background task will be splitted on to two parts - before foreground and after foreground.
- After the foreground task is done, background task will continue tracking time until done.