I don't think so. I see certain condition precedents for certain prophecies being fulfilled. For example, the nation of Israel has to come back into existance. There has to be a Third rebuilt temple that could be built in Jerusalem. This puts back on prophecies in Daniel, Matthew and Revelation.
Any times prior to these condition precedents reflecting Israel is already ruled out. So, any predictions prior to 1948 are overturned. Then you have Jews from all over the world returning back to Jerusalem, this forms another condition precedent. Any times prior to this is again ruled out.
Now, you are going to this millenium. You need a rebuilt Jewish temple to fulfill parts of Daniel 9. A third rebuilt temple. Part of the most convicing evidence out there now is to see what steps are being made to build this right now:
http://www.templeinstitute.org/
By the way, this is a Jewish source that has no knowledge of bible prophecy or how I would be interpreting this. Having a rebuilt temple is a condition precedent for the false prophet setting up an idol statue inside as it states in Revelation 13 and Thesselonians and Matthew and in Daniel. Four different parts of the Bible state the same thing. It's called the abomination that maketh desolate.
The Revived Roman empire, or what's shaping up there looks like geopolitical condition precedents for bible prophecy. Iran and other Arab nations going against Israel in some future war sounds like Ezekiel 26.
I can go on and on...but the fact is there are condition precedents that are shaping up that all point as signs that we are closing in to the last days. I can easily say that your other dates fail on some of their condition precedents and are invalid.
The concept of making up these new predictions of dates is simply based on noticing more condition precedents forming up for Bible prophecy fulfillment, and noticing a convergance from other sources indicating that something could be up. Sometimes science is used to explain anomalies in the Bible. If for example, the Bible predicts astronomical and planetary disruptions causing lots of death and wierd things -- then science may explain this as pole shifting caused by an unusual alignment between the earth, the sun and the centre of the milky way galaxy which contains a black hole -- the mayan calander appears to have some sort of authority that this alignment would take place on December 12, 2012 as it seems to know how the planet earth aligns itself relative to othe other stars. This aligment may also affect nearby asteroids, meteriors or brown starts to alter their orbits closer towards the earth. If Jesus said there are going to be earthquakes in diverse places, rumours of wars, famines, pestilence in greater frequency, and pole shifting or anomalies that occur due to these alignments, then it stands to reason that there is a convergence point.
Now, I'm not saying December 12, 2012 is going to coincide with the date with some of the biblical plagues and pestilence in Revelation, I'm just NOTICING a POTENTIAL CONVERGENCE around that time. I'm also NOTING that with Iran, and since 9/11 there are being geo-political changes, there are being economical changes in America, the world is not staying the same and it's moving in a direction that appears to be consistent with forming of condition precedents of a revived roman empire.
