David Bain (36) is on trial a second time for murdering his father, mother, two sisters and younger brother on June 20, 1994.
The Crown says Bain shot the family, entering in the computer what was meant to look like a suicide message pointing to his father as the killer.
But Bain denies committing the murders.
He says he returned from his paper round about 6.45am and, soon afterwards, saw his mother lying dead in her bed and his father, also dead, lying on the floor of the room next to the computer alcove.
The defence argues Bain could not have been the killer because the time the computer was switched on meant he would still have been on his way home from his paper round, that the 6.44am switch-on time given by a computer expert at Bain's 1995 trial was based on incorrect information.
That was because the watch worn by a police officer timing the various steps taken by a computer expert was two minutes fast, the defence say.
Police responding to Bain's 111 emergency call about 7.30am on June 20, 1994, found not only 58-year-old Robin Bain and his wife Margaret (50), dead from a single gunshot wounds to the head, but their three other children Arawa (19), Laniet (18), and Stephen (14), also shot dead.
On a computer screen in a curtained-off alcove in the lounge where Robin Bain's body lay near a rifle, was the message "Sorry, you are the only one who deserved to stay".
A University of Otago computer expert, Martin Cox, said he ascertained the switch-on time of the computer as 6.44am by saving the message at 2.16pm on June 21, and working back from that, using information from the computer's internal clock.
He was able to establish that 31 hours and 32 minutes had passed since the computer was turned on, taking him back to the 6.44am the previous day. That was the time accepted at Bain's first trial.
Mr Cox said it was not possible to say when the message was written on the computer.
It could have been entered any time after the machine was switched on.
It had not been saved when he inspected the IBM compatible machine, which he believed was about 10 years old.
The computer had a hard disk in it, something "a little unusual for a home machine".
From the information at the top of the screen, he could tell the message had not been saved and he saved it and undertook the process needed to calculate the time he believed the computer had been switched on.
Defence counsel Michael Reed QC challenged the 6.44am switch-on time.
Because Detective Kevin Anderson's watch was later found to be two minutes fast, that would move the start time back to 6.42am on June 20.
And that could be extended by another 54 seconds if subsequent testing by a police computer expert was correct, Mr Reed suggested.
Mr Cox agreed if that testing was correct, that could also make a difference to the time.
He said he first became aware of the importance of the switch-on time in relation to the David Bain trial "about this stage of the first trial".
But he had not been aware of the importance of the other side of the equation, namely the time Bain said he got home.
He said he was not cross-examined about his evidence.
The last witness called yesterday, Marie Fitchett, who was a detective in Dunedin in 1994, said she was told in the course of her inquiries that Robin Bain was "into computers in a big way" and that he had introduced computers into the Taieri Beach School, where he was principal, something "above and beyond what was normal then".
