they also said like last year that one could hit us in 2017.. a month later it was confirmed it wouldn't.
Remember they are so far away.. that they only have to change the way they are going like 0,01% and that would make them go like million miles past the earth or something like that.
Anyway if something big like that would hit the earth.. atleast we would have few years to prepare for it
It doesn't happen like it does in the movies that something suddenly hits the earth like a month after people found out... If it would then it wouldn't be big enough to destroy the planet.