As a new era of remote and in-office work begins, some companies are trying to bring definition to daily schedules—by making some hours off-limits for meetings.

The tactic, called “core hours,” sets times—say, between 10 a.m. and 2 p.m. or 1 p.m. and 4 p.m.—when bosses require employees to be online and available for Zoom meetings, project collaboration and other exchanges. Any other time is a meeting-free zone.

By having certain hours, or days, when everyone is “on,” the idea goes, employees have more freedom and flexibility to do solo work the rest of the time.

The approach—practiced by some employers over the decades as an effort to keep working parents from being boxed out of early morning or late afternoon meetings—was adopted by some bosses during the pandemic as a way to keep remote collaboration from bleeding into all hours of the day. Now, as businesses reopen offices or implement longer-term work-from-home strategies, some companies say they are making core hours standard practice.

San Francisco-based Dropbox, which is allowing workers to continue working remotely if they prefer, has set core collaboration hours for employees in the Americas between 9 a.m. and 1 p.m. Pacific Time. Those hours partly overlap with core hours set for Dropbox workers in Europe and Asia. Slack Technologies Inc., also in San Francisco, is pushing teams to limit meeting times to about four hours a day, letting groups decide which time windows work best for them.

Atlanta-based scheduling software maker Calendly restricted meetings to between noon and 5 p.m. Eastern Time after noticing that many were being scheduled at times that cut into its far-flung employees’ evening plans or early morning routines, Calendly Chief Executive Tope Awotona said.

“All we care about is that you get your job done from anywhere, you hit your goals, and you make yourself available during what we call our core hours,” said Mr. Awotona, whose company plans to let staff continue to work remotely yet still gather at times in co-working spaces.

“ ‘All we care about is that you get your job done from anywhere, you hit your goals, and you make yourself available during what we call our core hours.’ ”

— Calendly CEO Tope Awotona

That philosophy runs counter to the reality of many white-collar professionals. Smartphones, messaging apps, Zoom calls and other workplace technologies have made it possible for workers to be reachable and available nearly every hour of the day—and fueled many bosses’ expectations that they should be.

Many employees say remote work in the pandemic has blurred work-life boundaries even further, seeping into what used to be commute times, lunch breaks and other pauses that broke up the office-based workday. A National Bureau of Economic Research study of more than three million workers’ anonymized email and calendar data in 2020 showed the average workday lengthened by 48 minutes and the number of meetings increased by 13% after the pandemic’s onset.

Work schedules could become even more complicated in the coming months, as some workers come back into the office, others stay out and many work a mix of remote and in-office days. Some managers say core hours are a remedy for preserving employees’ productivity and sanity in hybrid work situations—and keep meetings from proliferating out of control.

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“If you give people from nine until five to schedule meetings, they’ll fill the day full of meetings,” leaving no time to get other work done, said Brian Elliott, vice president of Slack’s Future Forum, a consortium launched by the company to help businesses rethink the future of work.

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Before Slack set its own core-hours policy early on in the pandemic, Mr. Elliott said he, too, found it a struggle to squeeze in writing during the brief lulls between meetings. Now his team reserves the period between 10 a.m. and 2 p.m. Pacific Time for group huddles and other discussion, allowing him and his colleagues to get more done the rest of the day.

“I’ve got several people on the team who are primary caregivers, and it just was not physically possible for them to be present and to be active from 9 a.m. until 5 p.m.,” he said.

As part of Dropbox’s plans to let employees work from home indefinitely, the company said it surveyed employees and found that many didn’t just want flexible hours. They wanted the company to set boundaries around meeting times so the onus wouldn’t be entirely on them to “defend their schedules,” said Dropbox spokeswoman Jasmine Castro-Torres.

For Lewis Taylor, Dropbox vice president of customer experience, the shift has meant more freedom in his schedule and fewer meetings. He said he now finds time for walks with his dog or a quick break to help him recharge during intense workdays. “I can say, ‘OK, I have 30 minutes. After this, we can go outside and toss the Frisbee around,’ ” he said.

Yet some companies with far-flung workforces say establishing core hours for everyone can be too regimented. At FourKites Inc., a Chicago-based supply-chain platform provider with operations in Singapore and Amsterdam, executives looked into setting varying core hours for employees, depending on their region. They soon determined the practice would make it harder to meet project deadlines and make decisions quickly, said Christine Schoeff, FourKites’ chief people officer.

“The question of core hours gets really tricky,” said Ms. Schoeff. “We had to lean into trust and choice for employees, and really know what they need to get done.”

Rather than institute a companywide policy, some businesses are instructing teams to practice whatever approach works best for them. Megan Flanagan, vice president of field and partner marketing at cybersecurity firm Arctic Wolf Networks, said that over the course of the pandemic, her son’s preschool closed three times, leaving her to juggle caring for her toddler and video calls with colleagues. That became manageable, she said, after her manager and teammates decided to limit meetings to the child’s daily nap time, usually between noon and 3 p.m.

“If I didn’t have supportive colleagues, I wouldn’t have gotten through it,” said Ms. Flanagan, adding that her team has kept to that schedule. “Being able to flex my hours during those times when Jack was home ensured that I could apply the right focus on my work—and on my son.”