Quick answer

Only where the contract expressly provides: absent clear words the decision-maker cannot cut the contract period or withdraw an extension already granted to reflect omitted work, and no form may reduce the period below the original completion date

The answer is a matter of the contract's wording. The default position favours the contractor: having been given a period to complete, it should not have that period cut after the event to reflect omitted work, and the courts will not imply a power to do so. Several standard forms create a limited express power. The JCT form lets the reviewer fix an earlier completion date on the post-completion review to reflect omissions, but never earlier than the original completion date. Some government works forms let an interim extension be reduced to the extent of an authorised omission. A common engineering form forbids any decrease of an extension already granted. Other forms, including the main process and plant forms and the NEC, say nothing about omissions, so no reduction is available. Even where time cannot be taken back, saved time from omissions can be set against a later delay when a further extension is considered.