“I love deadlines,” wrote the British humorist Douglas Adams. “I love the whooshing noise they make as they go by.” On Tuesday, a deadline for Iran and six world powers to reach a deal over the Islamic Republic’s nuclear program whooshed by—for the fourth time since an interim nuclear agreement was signed in November 2013.

"We are interpreting in a flexible way our deadline, which means that we are taking the time, the days we still need, to finalize the agreement," European Union foreign-policy chief Federica Mogherini told reporters in Vienna. Mogherini, confusingly, said that the continuation of talks past the Tuesday deadline—which was set after the parties failed to meet an earlier deadline on June 30th—“does not mean we are extending our deadline,” only that negotiations were continuing.

Just to be safe, the United States announced that it had extended the terms of the 2013 interim nuclear deal with Iran until Friday. Despite the flexibility on timing, the nuclear negotiations have dealt in a lot of rhetorical absolutes, for instance Secretary of State John Kerry’s continuing threats to walk away if Iran doesn’t budge on key issues. Given this disconnect,  it’s worth asking what the deadlines actually mean in these negotiations, especially as they continue to be extended.

“There are different functions that an extension can play,” said David Holloway, a professor of political science at Stanford University and the author of Stalin and the Bomb. He added that deadlines in international negotiations are often pushed back because they were too ambitious in the first place or because the issues are hard to figure out. In the case of arms negotiations, Holloway also notes the potential impact of domestic politics on the parties participating in the talks. With Iran’s nuclear program in mind, securing a deal “could involve getting a final decision from the Supreme Leader or the White House.”

Beyond the negotiating table, the breaking of a deadline also serves a symbolic function. Should the negotiators fail to strike a deal this week in Vienna and pack up to return to their home countries empty-handed, there’s meaning (or at least spin) in the notion that the the talks went weeks beyond their allotted time.

“Extensions are a way of underlining that ‘we really pushed this as hard as we could,’” Holloway said, noting that the strategy can bear “a theatrical aspect or a performative aspect.”

But if a deadline is flexible, why have it at all? “Deadlines exist to try to force action,” said Don Moore, a professor at Berkeley’s Haas School of Business. “And without deadlines that don’t impose real cost on the other side, it can be hard to push a deal through.” Moore added that without a deadline, “each side believes that they can benefit from delay.”

Iran has sought to end a tough regimen of international sanctions that have hurt its economy; CIA Director John Brennan has credited this economic pain with bringing Iran to the negotiation table. Whether the sanctions have bit enough to force Iran into a final deal is yet to be understood. “The aim is to make the passage of time costly,” Moore explained.

But, he added, negotiations tend not to have a final deadline. (One hypothetical exception: “negotiating with a dying patient over the terms of a will.”) In lieu of a break-off point, what often decides when talks end are “time costs.” In other words, when waiting hurt one side more than another.

Moore relayed the story of the armistice negotiations between North Korea and the United States at the end of the Korean War. “The American delegation rented rooms in a hotel,” Moore said, while the Korean delegation “signed a two-year lease.” The message was clear. And the talks took two years, two weeks, and the death of Stalin to yield an agreement.