@granroth @Boundegar
I think it’s rather about measuring time than height.
It’s pretty cool. Someone can correct me but I think it is because the higher the clock is the longer the path it has to take during the earth rotation - hence its higher speed and consequently the time dilation.
Normally to measure the time dilation you had to accelerate the clock to high speeds, and here you just put the clocks in an attic and a basement and wait few years 
