How did you get that out of "From 1876 to 1897, Fort Sumter was used only as an unmanned lighthouse station." ?
Does not compute.
Does not compute.
1861-1876:Active military outpost (15 years)
1876-1897--Unmanned lighthouse station (21 years)
1897-1947--Active military outpost (50 years)
So, 65 years as an active military outpost. 21 years as an unmanned lighthouse station