WASHINGTON — Tens of thousands of military reservists may be eligible for extra vacation or nearly a month’s worth of pay from federal departments due to a recent court ruling concerning their past mobilizations.

The government’s bill for the windfall could stretch into the hundreds of millions of dollars, but few individuals eligible for the extra benefits have thus far pursued them. Advocates think that’s because the ramifications of the court ruling aren’t well known, despite their widespread potential impact.

“Some of the agencies involved with this are going to be facing pretty big bills,” said Anthony Kuhn, managing partner at the law firm Tully Rinckey. “So I don’t expect them to be happy about it or really promoting it.”

At issue are reservists who hold federal jobs and were mobilized to active-duty at any point since the terrorist attacks on Sept. 11, 2001.

Under federal law, those civil service employees are eligible for 15 days of paid military leave a year on top of other benefits. Previous lawsuits on that issue resulted in large numbers of reservists receiving extra days off or payouts for unused leave time if they had already left their government jobs.

And federal rules also include provisions for up to 22 additional days of paid leave annually for reservists who are activated in support of contingency operations. Individuals who deployed to Iraq, Afghanistan or other overseas locations in recent years have received those benefits without dispute.

But advocates have argued that reservists activated in support of those overseas operations who remain stateside should still be eligible for the extra leave time.

“Until recently, that issue was narrowly interpreted by federal officials,” Kuhn said. “You had to be sent into a combat zone to be eligible for those extra 22 days. This lawsuit changed that.”

The suit, O’Farrell v. Department of Defense, was finalized earlier this spring when a U.S. Court of Appeals ruled in favor of a broader interpretation of what counts as a contingency mobilization.

The plaintiff, Department of Defense employee Michael O’Farrell Jr., was mobilized as a reservist in 2013 to work at the Naval Surface Warfare Center in California for 162 days.

Because his assignment was connected to contingency operations overseas, the court ruled he was eligible for the 22 extra days of leave, even though Merit Systems Protection Board officials had originally denied the claim.

As a result, tens of thousands of reservists previously denied the additional leave may be able to push federal officials for the vacation time or other compensation. Reservists who work for private-sector businesses or were mobilized under non-contingency orders are not eligible for them.

Kuhn said his firm is doing a public awareness push on the issue, setting up new systems to process what they expect to be a flood of reservists looking into the issue. Even with the reduction in the tempo of military operations in recent years, mobilizations of reservists for a host of active-duty roles remains commonplace.