The end of the world is nigh; 21 May, to be precise. That's the date when Harold Camping, a preacher from Oakland, California, is confidently predicting the Second Coming of the Lord. At about 6pm, he reckons 2 per cent of the world's population will be immediately "raptured" to Heaven; the rest of us will get sent straight to the Other Place.

If Mr Camping were speaking from any normal pulpit, it would be easy to dismiss him as just another religious eccentric wrongly calling the apocalypse. But thanks to this elderly man's ubiquity, on America's airwaves and billboards, his unlikely Doomsday message is almost impossible to ignore.

Every day Mr Camping, an 89-year-old former civil engineer, speaks to his followers via the Family Radio Network, a religious broadcasting organisation funded entirely by donations from listeners. Such is their generosity (assets total $120m) that his network now owns 66 stations in the US alone.

Those deep pockets were raided to allow Family Radio to launch a high-profile advertising campaign, proclaiming the approaching Day of Judgement. More than 2,000 billboards across the US are adorned with its slogans, which include "Blow the trumpet, warn the people!". A fleet of logoed camper vans is touring every state in the nation. "It's getting real close. It's really getting pretty awesome, when you think about it," Mr Camping told The Independent on Sunday. "We're not talking about a ball game, or a marriage, or graduating from college. We're talking about the end of the world, a matter of being eternally dead, or being eternally alive, and it's all coming to a head right now."

Mr Camping, who makes programmes in 48 languages, boasts tens of thousands of followers across the globe, with radio stations in South Africa, Russia and Turkey. After 70 years of studying the Bible, he claims to have developed a system that uses mathematics to interpret prophesies hidden in it. He says the world will end on 21 May, because that will be 722,500 days from 1 April AD33, which he believes was the day of the Crucifixion. The figure of 722,500 is important because you get it by multiplying three holy numbers (five, 10 and 17) together twice. "When I found this out, I tell you, it blew my mind," he said.

Recent events, such as earthquakes in Japan, New Zealand and Haiti, are harbingers of impending doom, he says, as are changing social values. "All the stealing, and the lying, and the wickedness and the sexual perversion that is going on in society is telling us something," he says. "So too is the gay pride movement. It was sent by God as a sign of the end."

Mr Camping, who founded Family Radio in the 1950s, grew up a Baptist. Many of his strongly held views – he does not believe in evolution and thinks all abortion should be banned – are relatively commonplace among America's religious right.

Critics point out that this isn't the first time Mr Camping has predicted the second coming. On 6 September 1994, hundreds of his listeners gathered at an auditorium in Alameda looking forward to Christ's return.

"At that time there was a lot of the Bible I had not really researched very carefully," he said last week. "But now, we've had the chance to do just an enormous amount of additional study and God has given us outstanding proofs that it really is going to happen."