According to NJ.com, many believers in the 21st date say the Bible contains unquestionable clues. God tells Noah the world will end in seven days; the Bible also equates a day to 1,000 years. The date of the flood has been set at 4990 B.C., so adding 7,000 years plus one for the missing year "0" produces the year 2011. Translating a Biblical reference to a month and date, from the Hebrew calendar to the Gregorian, results in May 21.
But the Hebrews read right to left, so it's really the 5th day of the 21st month, and they have 21 months only on Uranus, so next month, the whole world gets blown out Uranus.