He was working on the machine the day United States president John F. Kennedy was assassinated on Friday, November 22, 1963.
Yesterday, he was again working on the International Computers and Tabulators ICT1301, only this time for a looming deadline for the reopening of the Toitu Otago Settlers Museum.
"My watch has probably got more power than this computer has," Mr McMillan said.
"It was used at Cadbury's from December 1963 until December 1975, when it was replaced and removed from service.
"We're hoping to be able to generate some lights and make the displays look like they're going again."
The computer was later gifted by Cadbury's to the Seacliff Museum of Transport and Technology, which later donated it to Dunedin city.
The ICT1301 needs 65sq m of floor space, weighs about five tonnes and looks like it was designed by steampunk enthusiasts.
The wiring is held together with waxed twine, rather than the plastic cable ties used today.
The computer used core memory, drum storage and punched cards and was popular at the time because it was based on decimal logic, so programmers did not have to learn binary or octal arithmetic.
"It was equivalent to about 6000 bytes. Nowadays, the information is all kept on discs and hard drives, but they didn't have that technology back then," Mr McMillan said.
The machine could read up to 600 cards per minute and was used for Cadbury's invoicing and payroll.
"There were four girls punching and four girls verifying. The girls were pretty fast."
The machine cost around 65,000, he recalled.
"At this time, the government was trying to discourage companies buying photocopiers and computers. You had to pay an extra third on top of the purchase price, which went into a bond and which you got back after six months."
About 200 of the machines were sold internationally, before the technology was superseded.
The redeveloped Toitu Otago Settlers Museum will be officially opened on December 7, with the public opening on December 8.
