Thanks for bringing up the subject of John Aquila Brown. No doubt the Adventist beliefs of William Miller, Charles Russell, and all the others would never have taken root without Brown.
I do think it has roots earlier than that. Not long ago I got a chance to look at a copy of Ussher's Chronology (reprinted in modern text) and it hit me that the time calculations of the Adventists were made possible by Ussher. It is a phenomenal book, wrong in places, but still quite an impressive book for one man to accomplish in the 1600's.