Michael Worthington’s job requires a drill, a microscope and a vast amount of accumulated knowledge. It requires something else, too: tact.

“There’s a lot of diplomacy in my job,” Worthington told Answer Man.

Worthington’s job is telling people how old their house is. How old it really is, not how old everyone thinks it is. Or hopes it is. Some people bristle when Worthington has finished his work and lops a few decades off the age of their building.

“With those people, you have to be a little kid-gloved,” he said. “You have to cajole them away from the idea they had and give them a new story. Once they have a new story they can hold onto, they are much more forgiving that their house isn’t the oldest building anymore.”

By what sorcery does Worthington discern when exactly a house was constructed? By looking at tree rings.

Changes in weather affect the width of each year’s tree ring. A ring grows wider in wet years, narrower in times of drought.

“Over a long period of time — 50-plus years — that becomes a unique sequence,” Worthington explained. “Any two trees growing at the same time will collect a similar sequence of wide and narrow rings.”

Worthington runs the Oxford Tree-Ring Laboratory in Baltimore and is a research associate at the University of Maryland. When he’s hired to date a building, he starts by finding original wood, such as in floor joists and wall posts. Using a drill, he takes six to 10 pencil-sized samples. He’s looking for pieces with more than 50 rings and a bark edge. (The bark edge shows when the tree was felled.)

Worthington has an archive of hundreds of samples of oak, yellow pine and tulip poplar for which he knows the date range of the rings.

“Hopefully, a few match and we can make a chronology for that building,” he said.

The science — called dendrochronology — is amazingly precise. It can estimate that a plank came from a pine felled, say, in the summer of 1780.

But couldn’t the lumber have been sitting around for decades?

No, said Worthington. Eighteenth-century carpenters didn’t have power tools. They liked working with green wood. It was softer, easier to cut and shape.

“A job that takes a day with green wood will take a week with seasoned wood,” he said. “No carpenter wants to do that.”

Dendrochronology started revolutionizing historical preservation on the East Coast in the 1980s. While historians once had to guesstimate from land records and architectural styles, now they had science.

“It’s really important to document with dendrochronology,” said Gretchen Bulova, director of the Office of Historic Alexandria.

Important, but occasionally disruptive. For years it was thought the oldest part of Gadsby’s Tavern in Alexandria was built in 1752, a date later adjusted to the 1770s. The tree rings told another story. Wood in the tavern was cut in 1785 — the spring of 1785 in the northern Shenandoah Valley, to be exact.

“There’s no going back,” Bulova said. “That’s a fact.”

Worthington worked on a series of projects in New England. His findings were later compared with what were previously believed to be the dates of construction. Dendrochronology proved that about half of the buildings were not as old as once thought. A quarter were about the same and a quarter were older.

Worthington prefers not to work with clients who are likely to get snarly if their house moves forward in time. And he insists his results be made public.

A few weeks ago, Answer Man wrote about the Murray-Dick-Fawcett House at 517 Prince St. in Alexandria. The city dates it to 1772. Some readers suggested that older houses exist in Alexandria, among them 415 Wolfe St. (1750, reportedly) and Captain’s Row on Prince Street (dating to the Revolutionary War, reportedly).

But according to “Historic Alexandria, Va., Street by Street,” a book written by Ethelyn Cox, the core of the Wolfe Street house is from 1788. The original Captain’s Row was destroyed by fire in 1827 and rebuilt.

Many historical houses just don’t survive in their original form, which is why 517 Prince St. is so remarkable. It is the oldest house that is most intact.

Altered houses have interesting stories, too. And dendrochronology can help tell them. Some of Worthington’s work involves sampling different areas in old houses to see how they changed over time: when a fireplace was moved or a new door put in.

Of course, answers to those questions just prompt more questions: Why was the fireplace moved? What was going through the minds of the people who moved it? And that’s why historians love old houses: for the stories of the folks who lived in them.

Twitter: @johnkelly

For previous columns, visit washingtonpost.com/john-kelly.