Was it only the 7000 year time period that pointed to May 21, 2011 as the Day of Judgment?
Not quite. We had known for many years already that May 21, 2011 would be the last day of the 8400 days of the exact 23 Great Tribulation period. The Great Tribulation started on May 21, 1988 and continued for 23 full years (8400 days) until May 21, 2011. We were long looking at that date as a possible end date for the world. The only problem we had (back then) was the additional information of the last day of the feast of tabernacles (which we thought would be the last day) occurring on Oct. 21, 2011. There was a 5 months gap between the two dates.
But once it was noticed that May 21, 2011 had the underlying Hebrew calendar date of the 17th day of the 2nd Hebrew month, then Judgment Day and the shutting of the door of heaven was locked into place by the converging nature of the end of the 23 years/8400 days Great Tribulation falling on that date and the underlying Hebrew calendar date that matched perfectly with the flood date for the shutting of the ark's door and the falling of the rain.
Some people, I'm sorry to say, count on man's forgetfulness when trying to diminish the Biblical evidence for May 21, 2011 being the Day of Judgment. They also forget that Mr. Camping was a very cautious man when it came to saying "this is what the Bible says" and would never have boldly proclaimed that date with an exclamation point unless the evidence was overwhelming and conclusive, which it was.