Someone asked the following question:
> I would like to understand if the changeover from the Julian to the Gregorian
> calendar in 1752 (sic) was accounted for in computing the date of May 21, 2011?
The short answer is absolutely! All calendar days have been accounted for and none have been overlooked, so all the dates appearing in Mr Camping's books are correct.
Explanation: the reason for the Gregorian reform was because the Julian calendar year was too long, i.e., 365.25 days/year (i.e., 100 leap years per 400 years) while an actual solar year is ~365.24219 days. So what happened is by the year AD 1582, the spring equinox (used to determine Easter) according to the Julian calendar had shifted backwards by 10 days, so instead of the equinox occurring on March 21 it appeared to occur on March 11! So the calendar adopted under Julius Caesar in 45BC to track the seasons became obviously inaccurate 1600 years later. In order to correct this error introduced by the "long" Julian calendar year, the Roman bishop Gregory adopted a change in which Oct 4, 1582 would be followed by Oct 15, 1582. This change corrected the misalignment between the accounting calendar and true solar calendar so that the vernal equinox of the following year, AD 1583, occurred on March 21 as it typically does.
The second part of the Gregorian calendar reform was that the number of leap years was changed from 100 days per 400 years to 97 days per 400 years, allowing the new calendar to better track the true solar calendar (only a 58 minute error every 400 years relative to the vernal equinox tropical year). This small error relative to the vernal equinox, even when applied over the almost 2000 years since Christ, results in less than a 5 hour difference between the actual seasonal/solar calendar and the proleptic Gregorian calendar we use to date 1st century events. So does this 5 hour difference over 2000 years result in any problems? The answer here again is no, because we don't actually use the Gregorian calendar itself to determine when events in the first century occurred. Instead "we" (i.e., NASA) use astronomical time (i.e., Terrestrial Dynamical Time) to determine when the vernal equinox and new moons of the 1st century occurred, and we (Bible students) use their data to determine historical Biblical events (e.g., the date of Jesus' baptism, crucifixion, resurrection, and Passover AD 33). To make our job easier, NASA converts from astronomical time to universal time (GMT), so we have exact and accurate dates and times for the astronomical events (vernal equinox and new moons) relative to the proleptic Gregorian calendar so we can be sure our dating of first century Biblical events is accurate.