It's a complicated business calculating the precise date of the end of the world.There's the Great Flood to consider, which may have happened around 4990BC, depending on who's estimating. And the timing of the exodus of the Jews from Egypt. Then there's a bit of maths that involves equating one day to 1,000 years.

Do all that and it turns out that Armageddon will begin at 6pm on Saturday. That is, if Harold Camping has got his calculations and his reading of the Book of Ezekiel right.

The 89-year-old doomsday prophet, a former engineer who perhaps inevitably comes from California, has prompted a tide of expectation, elation and derision after persuading listeners to his Family Radio Worldwide across the US and as far away as the Philippines to sell up everything and prepare for the beginning of the end of the world with the second coming of Jesus.

If all goes according to plan, those who have been "saved" by Jesus will rise into the air in the Rapture and look down as God smites billions of nonbelievers with a great earthquake rolling from city to city across the planet, and a bit of fire to boot.

"Everyone will be weeping and wailing because they'll know in a few hours it'll come to their city," Camping told the TYT Now online news show. "It's going to be a horror story of tremendous proportion."

Judgment day will begin at 6pm wherever you are. The mayhem will move west over the planet, wiping out cities, towns and villages.

In the US, some believers have given up their jobs and donated money they think they will no longer need to pay for more than 2,000 billboards across the country proclaiming "Judgment Day: May 21, 2011 – Cry mightily unto God. THE BIBLE GUARANTEES IT!"

Thousands of people, some wearing T-shirts proclaiming that doomsday is at hand, have said goodbye to family and friends. It is not always welcome. Abby Haddad Carson gave up her job as a nurse two years ago to spread the message. Her three children do not believe it. "My mom has told me directly that I'm not going to get into heaven," Grace Haddad, 26, told the New York Times. "At first it was really upsetting but it's what she believes."

Callers to Christian radio stations have debated what to do about nonbelieving friends and neighbours who will be left behind to endure the wrath of God.

One caller in Oregon wanted to know if he should arm himself to protect his family from the doomed in his street who might be jealous that those who have "found Jesus" were about to go to heaven.

The show's host assured him that nonbelievers would be too busy being tortured by fire to worry about seeking vengeance on him. Even some nonbelievers are getting in on the act. Atheists are throwing "after Rapture" parties to celebrate the departure of the religious – or at least Christians – from their midst.

The Washington DC transportation department put out a tweet saying that the coming apocalypse would have an impact on road maintenance: "Sorry, we will no longer be able to fill your potholes after Saturday." An enterprising New York business is offering to take care of the cats and dogs of those who believe that their Lord will take them to heaven without their pets.

Camping previously predicted that the end of the world would be in 1994. He blames that on an error in the maths but says he has it right this time. "There is no possibility that it will not happen because our information comes from the Bible," he told the Philadelphia Daily News.

Besides his mathematical formula, Camping has conjured up more "evidence" that doomsday looms. He has pointed to the re-establishment of Israel – which some Christians believe is a prerequisite for the second coming of Jesus and the Rapture – as a sign from God "that the world is getting near its end". He has said that the earthquake and tsunami in Japan were a divinely organised foretaste of what awaits most of humanity.

Camping has also said that "gay pride" and same-sex marriage are "a sign from God that judgment day is very near". "No sign is as dramatic and clear as the phenomenal worldwide success of the Gay Pride movement. In the Bible God describes His involvement with this dramatic movement … We will learn that the Gay Pride movement would successfully develop as a sign to the world that Judgement Day was about to occur," he writes.

Camping isn't planning anything special. He intends to stay at home and watch the mayhem. Not so most of his eight children. Only one believes their father's claim.