Camping's Biblical study regarding time and Christ's second coming is based on the cycles of:
Jewish feast days in the Hebrew calendar, as described in the Old Testament,
the lunar month calendar (1 synodic month = 29.53059 days), and
A close approximation of the Gregorian calendar tropical year (365.24219 days, rounded to 365.2422 ).
He projects these into modern times and combines the results with other information in the Bible.
Camping calculates date of the crucifixion of Christ as Friday April 1, AD 33. Not all commentators agree with that date. Hoehner argues for April 3, 33 A.D. Other students of the subject have placed the event in AD 29, 30, or 31.
Camping further calculates that the Rapture is 722,500 days after the crucifixion of Christ.From April 1 to May 21 is 51 days. Additionally, multiplying using the same math Camping uses returns an approximation of the remaining days. Using his date of April 1, AD 33, a total of 1978 years multiplied by 365.2425 days/year (Gregorian calendar) results in 722,449.66 days. Multiplying by 365.2422 (tropical year, seven significant digits) results in 722,449.07 days. Multiplying by 365.24219 (tropical year, eight significant digits) results in 722,449.05 days. If one accounts for leap seconds, each year is slightly longer than the previous. Since 1972 to 2011, the earth rotates 24 seconds slower. This is a difference of 0.000278 day over 39 years, for an average of only 0.000007 day per year.
In 1992, Camping published a book titled 1994?, in which he proclaimed that Christ's return might be on September 6, 1994. In that publication, he also mentioned that 2011 could be the end of the world. Camping's predictions use 1988 as a significant year in the events preceding the apocalypse; this was also the year he left Alameda Bible Fellowship. As a result, some individuals have criticized him for "date-setting. Camping's latest publications, We are Almost There! and To God be The Glory, refer to additional Biblical evidence which, in his opinion and that of others mentioned by him, pointed to May 21, 2011 as the date for the Rapture and October 21, 2011 as the date for the end of the world.
In an article "Is Harold Camping and Family Radio a Cult?", the evangelical Got Questions Ministries opposed Camping's teachings because they believe his entire method of Bible interpretation is flawed:
"Harold Camping employs an allegorical method of interpreting Scripture. Because of this method, the meaning of any Scripture passage is purely subjective, subject to the mind and imagination of the person. … Camping's use of an allegorical method of interpretation for Scripture, and especially for unfulfilled prophecy, is fatally flawed. It undermines the very nature of communication. God gave us His Word to communicate very specific information.