Gday yadda yaddda, Let me clarify the SDAs get 1844 from daniel 8.14 'Unto two thousand three hundred day then shall the sanctuary be cleansed 2300 days. That is converted to years from 457 bc the starting point for the seventy weeks in daniel 9 and the calculation comes to 1844.
the three and a half years is a separate prophecy spanning from 538 to 1798 and is 1260 days converted to years. this timespan is found as 42 months, time times and a dividing of time all in revelation. This prophecy is the reighn of the papacy when they persecuted the protestants and ended in 1798 when the papacy received a fatal blow the pope was arrested by general berther.
Of course this is the official historist view but theologians have different views suggesting a more futurist approach. To many SDAs the attack on the Pope as a product of the last century where most protestants had similar views that the Pope was the antichrist .
Barry