When I was reading the Landau's The Classical Theory of Fields, I found that when distance of four dimentional space is negative, the time's square should be a negative as well.Then time is an imaginary number. it's SPACELIKE. But what is imaginary time mean on earth? They say that in that case(spacelike), an event could be BEFORE or AFTER or SIMULTANEOUS with another event in the book. As far as I know, two imaginary number can't be compared. Then how is the time compared?