The Crown had filed a memorandum advising that "difficulties have arisen with the trial date because it clashes with the Islamic holy month of Ramadan".
Ramadan occurs over the month of May next year, affecting many of the trial's witnesses.
Survivors and victims of the March 15 shooting could still be abroad on a Hajj pilgrimage bankrolled by the Saudi king, the court heard at an earlier hearing.
Tarrant's defence team, consisting of Auckland lawyers Shane Tait and Jonathan Hudson, had no objection to the change in date.
The alleged killer remains in custody, while the case is next due to be called on October 3 when legal arguments to change the trial's venue are expected to be made by Tait and Hudson.
At this stage the trial is still set to be held in the High Court at Christchurch.
A transfer to another city would be permitted if the judge "is satisfied that it is in the interests of justice that the proceeding be heard at that other place or sitting", according to the Criminal Procedure Act.
Meanwhile, a Royal Commission of Inquiry headed by Justice Sir William Young of the Supreme Court is investigating the build-up to March 15.
The inquiry began within a month of the massacres – on April 10 – and will explore whether police and security agencies should have acted earlier.
It will also look at how the accused shooter acquired guns, whether relevant agencies could or should have known about his activities and whether they could have stopped the attack.
A perceived lack of official response to complaints about racial attacks are among the main issues which have been raised during the submission period thus far.
The role of social and mainstream media creating division has also been discussed.
Representatives from the Islamic Association, the New Zealand Police and the Institute for Law have presented to the inquiry, as well as dozens of public agencies including Ethnic Affairs, the Defence Force and New Zealand's intelligence agencies.
The findings are expected to be delivered on December 10.
