Incorrect. A prophet is called by God and does proclaim future events. This gives them credence as to Who their calling is from, but their main job is to call back a wayward people to repentence. In the case of the Cleansing of the sanctuary, no one was prophesying that.
That had already been done by Daniel. William Miller, as well as a few other individual who aren't as well known for beliving this, simply followed their understanding of said prophecy and taught others about it.
What a "great disappointment" he was ....
http://en.wikipedia.org/wiki/William_Miller_(preacher)
Although Miller was convinced of his calculations by 1818, he continued to study privately until 1823 to ensure the correctness of his interpretation. In September 1822, Miller formally stated his conclusions in a twenty-point document, including article 15: "I believe that the second coming of Jesus Christ is near, even at the door, even within twenty-one years,--on or before 1843."
[9] Miller did not, however, begin his public lecturing until the first Sunday in August 1831 in the town of Dresden.
[10]
After the failure of Miller's expectations for October 22, 1844, the date became known as the
Millerites'
Great Disappointment.
Hiram Edson recorded that "Our fondest hopes and expectations were blasted, and such a spirit of weeping came over us as I never experienced before... We wept, and wept, till the day dawn."
[15] Following the Great Disappointment most Millerites simply gave up their beliefs. Some did not and viewpoints and explanations proliferated.
Miller initially seems to have thought that Christ's Second Coming was still going to take place—that "the year of expectation was according to prophecy; but...that there might be an error in Bible
chronology, which was of human origin, that could throw the date off somewhat and account for the discrepancy."
[16]
Miller never gave up his belief in the Second Coming of
Christ; he died on December 20, 1849, still convinced that the Second Coming was imminent.
.