You may not have it marked on your calendar, but this coming Sunday is "adoption day." It's the day Iran must begin sharply curtailing its nuclear program as part of the landmark nuclear agreement reached this summer.
Nonproliferation experts say the steps Iran is about to take will put it significantly further away from having a nuclear weapon. Critics, however, warn of the possibility of cheating.
Iran doesn't see a need to call attention to adoption day. For Tehran, the point of the nuclear accord is to get out from under an extraordinary web of international economic sanctions. The price it agreed to pay — sharply reducing and opening up its nuclear program — is much less popular.
Iran's President Hassan Rouhani would rather focus on the benefits of rejoining the world economy, as he did at a rally earlier this year.
"With the final agreement — which, if the other side has a serious will, in the coming months will be possible — the situation of manufacturing in Iran will be much improved," he told a cheering crowd.
Rouhani also predicted the end of Iran's sanctions-driven black market, saying, "The dealers of sanctions should think of another job, from right now."
But what's actually starting now is something very different. Iran has to make big changes at three key nuclear facilities: the nuclear fuel enrichment sites at Natanz and underground at Fordow, and at the Arak plutonium reactor.
Mark Fitzpatrick of London's International Institute for Strategic Studies says these will be three momentous tasks: "One is removing about 14,000 centrifuges from Natanz and Fordow," he says. "Another is getting rid of almost 12,000 kilograms of low-enriched uranium, either shipping it out to Russia or diluting it to a natural state. And the third is removing the core of the reactor at Arak and filling it with concrete."
Fitzpatrick says that last step — removing and disabling the Arak reactor's core, known as a calandria — is, for practical purposes, irreversible, given the scrutiny Iran will be under.
"Iran has no incentive at all to replace the calandria at Arak unless they were hellbent on having a plutonium path to nuclear weapons," he says. "So it's very good that they've agreed to have a different kind of reactor so that they can't produce plutonium. It's a very big nonproliferation step."
The country will also be left with just a small fraction of its previous stockpile of enriched uranium, the other main type of nuclear fuel.
Iran will have to submit to intrusive inspections by the International Atomic Energy Agency (IAEA), aimed at ensuring that no nuclear material is diverted to a covert weapons program. Iran says it never had such a program, but that claim is deeply doubted in the West.
The question may be at least partially answered on another key date, Dec. 15.
That's when the IAEA is due to report on the possible military dimensions of Iran's past nuclear activities. The IAEA says Tehran is addressing some of the questions about past research, but critics say anything less than a complete explanation of all of Iran's past nuclear work would weaken the agency's ability to measure Iran's future nuclear work.
When they look further down the path of implementing this agreement, nonproliferation experts see potential pitfalls — possibly in the form of new congressional sanctions on Iran or a new American president who's not inclined to follow through with the Iran deal.
Iran's Foreign Minister Mohammad Javad Zarif addressed that last concern on a visit to New York last month, saying political changes in Washington should have no effect on the American commitment to living up to its commitments in the nuclear accord.
"I can say that in accordance with the international law, once the government of any country makes a commitment, the next government also has the obligation to accept and take on and continue to respect that commitment," Zarif said. "And the U.S. government has already accepted to stop sanctions on Iran."
Fitzpatrick also warns of unforeseen developments. What if, for instance, the Lebanese Hezbollah militia used Iranian-supplied missiles to attack Israeli cities? In that case, he says, anger at Iran could easily boil over into retaliation that would derail the deal.
And what about the sanctions relief that Iranians are waiting for? That will come on "implementation day," an unspecified date that can only be set once the U.N. has certified that Iran has met its nuclear obligations. Analysts predict that won't happen until late winter or even spring.