According to a report in Politico, the US Justice Department sought the delay from the court on Monday (US time), just a day before the hearing set down by Magistrate Judge Sheri Pym.
The agency said it may not need Apple's assistance to unlock the phone if the method for unlocking the iPhone demonstrated to it by an outside party was successful. It has been given until April 5 to test this method.
The FBI is looking to gain access to the phone which belongs to the San Bernardino County Department of Health and was being used by one of its employees, Syed Rizwan Farook, who was involved in an attack that killed 14 people in December last year. Farook also died in the process.
The FBI initially obtained a court order asking Apple to create a modified version of its iOS mobile operating system to remove a limit on the number of times one could try to guess the pass code and also the time intervals between guesses. When Apple refused, the agency came back with an order compelling Apple to fall in line.