Dr. Mills spent more than three decades as a professor at the University of Delaware and was active in designing key parts of the early internet in the late 1970s and 1980s. He was a lifelong contributor to open-source software, building tools that went on to be used and modified by engineers and tech companies to this day.
His dominant contribution was teaching computers how to tell the time.
In the 1970s, researchers were building out the Arpanet, an early, government-sponsored version of the web that connected various nodes at universities around the country. As the net grew and more machines were connected to it, the lack of a system to make sure they all had the same concept of time was beginning to cause problems.
Because there was an unpredictable time lag whenever one machine communicated with another, simply timestamping bits of code that went between computers was not good enough to keep things in order. It was a problem that had to be solved if the internet was going to be used for financial transactions, real-time communication and a million other potential applications.
As a researcher at Comsat, the company founded by the government to develop satellite communication networks, Dr. Mills had the chance to work on Arpanet, which had been built by the Defense Department’s Advanced Research Projects Agency. Dr. Mills began working on ways to sync computer time, partly because no one else was doing it, allowing him to do the project on his own terms, he told the New Yorker in 2022. In the late 1970s he invented the Network Time Protocol, known forevermore to programmers as NTP.
Part of Dr. Mills’s insight was to build a system that ranked various computers in a network by how reliable their concept of time is. Computers connected directly to an atomic clock are deemed the most reliable, and other machines in the network rapidly communicate with each other to determine a consensus on what time it is, through a mix of complicated math and clever programming.
“I remember being totally astonished,” Vint Cerf, a computer scientist who helped lead the development of the early internet and a close collaborator with Dr. Mills, said in an interview. “It was black magic.”
David Lennox Mills was born in Oakland, Calif., on June 3, 1938. His father was an engineer, founding a company that built oil seals for car engines, according to the New Yorker.
The younger Mills was born with glaucoma, had poor eyesight for much of his adult life and went blind in the years before his death. Cerf remembered Dr. Mills using a telescope to look at whiteboards. “He was not shy about the fact that he had a vision impairment,” Cerf said. “A very down-to-earth, cards-on-the-table kind of guy.”
Dr. Mills received five degrees from the University of Michigan, culminating in a doctorate in computer and communication science in 1971. He then taught computer science at the University of Edinburgh in Scotland and at the University of Maryland before joining Comsat in 1977.
In addition to his daughter, of Lebanon, Pa., survivors include his wife of 59 years, the former Beverly Csizmadia, of Newark; a son, Keith Mills of Jackson, Miss.; and a brother.
Beyond Network Time Protocol, Dr. Mills contributed to key parts of the original internet structure. His “fuzzball” software was used to run the first internet routers. The name was a Mills invention, too, because he saw them as helpful little critters, Cerf said.
It was just one example of what computer scientists came to call “Millsspeak.” Trustworthy clocks were called “truechimers,” while unreliable ones were “falsetickers.” A computer that overwhelmed an NTP hub with too many requests was quieted with a “kiss-o’-death packet.”
“It is an open secret among my correspondents that I on occasion do twitch the English language in mail messages and published works,” Dr. Mills wrote on his personal website. “If you read my papers or my mail, you know my resonances. If not, you can calibrate my naughtimeter from children’s books, outhouse walls and old English slang.”
He taught at the University of Delaware from 1986 to 2008 and continued to update the NTP code for decades, even as the internet became a central piece of human civilization. Every day, millions of computers synchronize their clocks seamlessly billions of times, constantly communicating to enable financial trades, Zoom meetings and power grid surges.
The system can sync clocks down to microseconds, or a millionth of a second, Julian Onions, a postdoctoral researcher at the University of Nottingham, said in a 2022 YouTube video explaining NTP.
Other contributors beyond Dr. Mills eventually took over more responsibility for upgrading the NTP system. In addition, Big Tech companies such as Google and Amazon have made their own updates to NTP that have become standard because of those companies’ power over the internet.
“It’s still one of the fundamental protocols of the internet,” Cerf said, and a piece of technology that cemented Dr. Mills’s position as an internet pioneer. “He was part of that pantheon.”
