Tasks that are indefinitely put on hold cannot simply be moved to a later date for consideration and should not have progress updated to 100% as they would simply fall off the radar...instead I would propose that an option was made available to suspend a task, putting perhaps a strikethrough the task label name, freezing it at it's current state of progress and rendering it immobile (i.e. task cannot be moved or extended). That way when you come to revisit at a later date, it will remain suspended in the state you left it in and you can simply pick up where you left off by 'un-suspending' it...