The measurement began after Franklin Delano Roosevelt entered office amid the tumult of the Great Depression. With banks caving in and jobs vanishing, FDR set to work passing laws and establishing new government bureaus to curb the economic suffering.
He swore in his entire Cabinet at once, signed 76 bills into law, and began rolling out the New Deal in his first 100 days in office -- a frenzy of activity that, ever since, all presidents have been matched against.
Presidents usually chafe at the comparison to FDR, who benefited from a Congress that could hardly say no at a time of economic calamity. But the 100-day construct abides, fueled both by journalists eager for a yardstick to measure a new administration and by presidents themselves, who lay out 100-day plans as candidates and almost unfailingly fall short.
On Saturday, President Donald Trump reaches his 100-day mark with a mix of anxiety and dismissal. He embraced the concept during his campaign, spelling out a plan for the opening stretch of his presidency that included repealing and replacing the Affordable Care Act, funding the construction of a border wall and passing a $1 trillion infrastructure measure.
But as the date nears, Trump finds himself straining under the same pressures his predecessors faced to demonstrate a raft of achievements in the opening sprint.
"No matter how much I accomplish during the ridiculous standard of the first 100 days, & it has been a lot (including [Supreme Court]), media will kill!" Trump tweeted Friday.
He's not the first president to downplay the 100 days notion, or at least tamp down expectations for the pace of his accomplishments.
"All this will not be finished in the first 100 days. Nor will it be finished in the first 1,000 days, nor in the life of this administration, nor even perhaps in our lifetime on this planet. But let us begin!" said President John F. Kennedy, who ordered the failed Bay of Pigs invasion 87 days into his term.
President Ronald Reagan was shot in the chest on his 69th day in office, and had to sign his first bill into law over a breakfast tray at the George Washington University hospital. Nonetheless, he delivered an address to a joint session of Congress on the eve of his 100th day mark, making an argument for cutting taxes and curbing inflation.
Aides to President George W. Bush, whose transition period was stalled after a recount and court battle over ballots in Florida, argued his 100-day assessment should be delayed. The media ignored the request and Bush marked the occasion by inviting all 535 members of Congress to the White House for lunch.
"Doing pretty darn good," was how Bush assessed his opening stretch to ABC's Charlie Gibson. "I think we've laid a foundation for some serious change in Washington -- first of all, a change of attitude."
President Barack Obama attempted to extend his evaluation period even before he was elected to the White House.
"It's probably going to be the first 1,000 days that makes the difference," he told a Colorado radio interviewer in 2008.
The report cards came anyway on April 29, and Obama convened a rare prime-time news conference in the East Room to tout his accomplishments.
"I have a much longer time horizon than I think you do when you're a candidate or if you're listening, I think, to the media reportage on a day-to-day basis," Obama insisted as he took questions on the H1N1 swine flu, the global economic crisis and foreign hotspots like Iraq and Pakistan.
If presidents have pushed back on the 100-day marker as an arbitrary and unrealistic measurement, most have taken solace -- perhaps misplaced -- that the intense scrutiny does, eventually, wane. Asked in 1993 whether President Bill Clinton would do anything differently during his second 100 days in office, spokesman George Stephanopoulos joked: "He's not going to count the days, first of all."
The moment of relief was short-lived.
"We are," replied one of the reporters in the White House briefing room.