The SDA church still holds to the date of 1844 but when jesus didn't come they then believed in the Shut Door in which probation was closed to all exept the little flock or the Adventist believers. They held to this for seven years and then decided it didn't promote church growth. This seven years was also referred to the tarring time .
The 1844 was later about the 1860s reinterpreted as the cleansing of the heavenly santuary [Daniel 8.14] where Jesus moved from the holy place to the most holy as the high priest does once a year on the day of atonement. This according to Adventists also marks the final atonement in heaven and is the start of the investigative judgement where the records in heaven are investigated to see who is worthy of the benefits of christs atonement.
The Adventists have never said Jesus came to earth invisibly this is a second Adventist construct.
Of course these theorys are not biblical and most Adventist theologians know this and many church members do also. I know a few Adventist ministers who don't believe the investigative judgement when they teach it they just teach it as church history. The German Adventists don't even teach it they believe its just an American thing.
Funny with many Adventist you can believe in evolution, disagree with the trinity and disagree with th evirgin birth but belief in the investigative is manditory because it is the one doctrine other churches don't have and identifys the Adventists as special. Barry