That way, they can be held at the same time as presidential elections scheduled in March.
Mugabe has indicated he will stand for re-election.
Critics say Mugabe could use the legislation to put a preferred successor into position.
The opposition Movement for Democratic Change unexpectedly voted in favour of the bill when it was passed by parliament in September, saying they were backing the changes as a "confidence-building measure".
The government and opposition are in talks to help end Zimbabwe's political and economic crisis, triggered by violent, government-sanctioned takeovers of white-owned farms.