The Family and Medical Leave Act celebrated a milestone birthday in February, but employers still struggle to comply with the 25-year-old law.

Employers often get tripped up on core elements of the law, including notifying employees of their rights to take FMLA leave — in some instances, without them even officially requesting it — and the documentation they can ask employees for to support any leave requests, legal experts say.

The FMLA — signed into law by President Bill Clinton on Feb. 5, 1993 — allows eligible employees to take up to 12 work weeks of unpaid leave during any 12-month period to attend to a serious health condition of the employee, parent, spouse or child, or for pregnancy or care of a newborn child, or for adoption or foster care of a child.

Employers are obligated to provide FMLA leave if workers report to a site with 50 or more employees within a 75-mile radius.

Jeff Nowak, Chicago-based co-chair of the labor and employment practice group for Franczek Radelet P.C., said the FMLA “has withstood the test of time to providing job-protected time to people who need it when they are severely injured, or their family members are injured or ill.”

“It is critical employers carefully administer the FMLA and become acutely aware of its rules, because those rules often trip employers up, thereby creating liability,” he said.

New York attorney Eric M. Nelson said the impact of the FMLA has been enormous, noting that at least three states, including New York, now have paid leave laws, New York City in November 2017 expanded its paid leave law, and “the concept now of paid family leave is on the president’s agenda.” “In my experience, employers aren’t as well versed as they should be in when they must allow or can deny leave,” Mr. Nelson said.

“They’re also often unsure what they can ask for in the way of notice from employees seeking leave, and what they can require from employees in the way of documentation to support a request for leave.”

Mr. Nelson added that best practices for employers include making sure their human resources personnel have both training and available resources regarding what is required of the company in connection with the employee’s request for, and the company’s rights regarding, leave.

Matthew Gomes, a partner with Weinberg, Wheeler, Hudgins, Gunn & Dial L.L.C. in Atlanta, said when a human resources manager or supervisor learns of an employee with a potentially serious health condition, “the employee doesn’t have to say, ‘I would like to take FMLA leave.’”

“You may have an affirmative duty to inform the employee of his or her rights under the FMLA,” Mr. Gomes said. “There have been a number of FMLA claims that have arisen in that circumstance because the employee says, ‘you the employer should have let me know that I could’ve taken FMLA leave and by not doing that you denied my FMLA rights as a result.’”

Stacie Caraway, employment law attorney with Miller & Martin P.L.L.C. in Chattanooga, Tennessee, describes FMLA documentation as “an employer’s FMLA prenup defining your FMLA marriage.”

“The big takeaway is to make sure your FMLA leave policy and your process for reviewing and approving FMLA leave paperwork has been vetted by legal,” Ms. Caraway said. “Employers have to think about what does the paperwork actually say. If there’s something on the FMLA leave paperwork that doesn’t make any sense or is asking you to approve leave on an ‘as needed’ basis, you have a seven-day period to get corrected paperwork once you tell the employee in writing that there’s a problem, and if they don’t correct it you can deny the leave.”

Barbara Hoey, a partner with Kelley Drye & Warren L.L.P. in New York, said employers “have a legal right to push back if the initial documentation is not sufficient, and the reason I have clients do that is once you’ve certified it, you’re stuck. You’ve done it, you’ve certified it, and now your rights are much more limited.”

Maria Rodriguez, partner and Western region head of the employment group with DLA Piper in Los Angeles, advised that the “employer’s policy needs to be clear, it needs to be distributed to all employees, and it needs to model the FMLA and any related state laws.”

“Intermittent leave of absence may qualify as FMLA or the equivalent law, so managers need to be trained to work with human resources so that when employees are taking time off on some sort of a regular or consistent basis, they need to be trained to make sure those employees are working with HR,” she said.

“You see a lot of abuse where people have this condition that flares up on Monday mornings and Friday afternoons,” Mr. Gomes said.

Debbie Friedman, member of Cozen O’Connor in Philadelphia, warned against automatically firing an employee if he or she does not return immediately to work after the 12-week leave has ended.

“This law can’t be viewed in a vacuum,” she said. “An employer has to carefully evaluate whether an employee is entitled to additional time off under state or local laws or ordinances or pursuant to company policies on leaves of absences.”