When "-것 같다" is used to show an assumption or guess for the past event, it is used with the ending "-던", showing for event of the past .
어제 철수는 집에 있-었던 것 같아.
It seems that 철수 was at home yesterday.
오늘 철수는 집에 있-는 것 같아.
It seems that 철수 is at home today.
내일 철수는 집에 있-을 것 같아.
it seems likely that 철수 will be at home tomorrow.