It's just another example of pseudo-religious bunk by people trying to interpret the bible to fit in with their own crackpot ideas. This time the perpetrator is Harold Camping. Here is his 'calculation':
"The number 5, Camping concluded, equals "atonement." Ten is "completeness." Seventeen means "heaven." Camping patiently explained how he reached his conclusion for May 21, 2011.
"Christ hung on the cross April 1, 33 A.D.," he began. "Now go to April 1 of 2011 A.D., and that's 1,978 years."
Camping then multiplied 1,978 by 365.2422 days - the number of days in each solar year, not to be confused with a calendar year.
Next, Camping noted that April 1 to May 21 encompasses 51 days. Add 51 to the sum of previous multiplication total, and it equals 722,500.
Camping realized that (5 x 10 x 17) x (5 x 10 x 17) = 722,500.
Or put into words: (Atonement x Completeness x Heaven), squared."
In other words, it's total numerological nonsense. Add, subtract and multiply various numbers until they give the result you want.