For a 4 week task the elapsed time may well be 3 weeks, but this does not mean that there is one week left until task completion.
The initial work may have been held up for all sorts of reasons.
It’s important to remember that it is the amount of effort that has been expended that counts and not the elapsed time.
From the above example, the amount of effort left may be more than one man week (assuming one person on the job).
From careful consideration of the remainder of the task a manager should be able to tell how much effort is still required until completion.
From this the remaining time for the task can be calculated. This assumes the resource level stays the same.
The remaining time may be 2 weeks or 3 weeks if one person remains on the job.
The project manager may try to add resource to bring the remaining time back to 1 week.
If this is unsatisfactory then either it will complete late or you may be able to reduce the scope.
The latter action should not be considered lightly and should be agreed by the stakeholders.
Under PRINCE2® [see ‘The Complete Project Management plus PRINCE2’] additional areas affected would be benefits and risk.
PRINCE2® is a Registered Trade Mark of the Office of Government Commerce in the United Kingdom and other countries.