No predictions loaded for 1991. This site serves NOAA predictions from its own store, and this period has not been fetched for Fort Sumter yet. Nothing is estimated or interpolated in its place. NOAA's own page for the station has the official table: tidesandcurrents.noaa.gov.
tidealmanac.site/us/sc/fort-sumter-8665728/1991
Fort Sumter, SC tide table 1991 (printable) · Tide Almanac