Firstly, I wish to thank you indeed very much for the effort you put into answeing this question. I downloaded the data from Xena browser
https://tcga.xenahubs.net/download/TCGA.GBM.sampleMap/GBM_clinicalMatrix
The problem was due to the wrong column:
CDE_survival_time
I chose, instead of the correct one:
days_to_death
Tanks again