I don’t know what they do on the backend to create this though.
They have the code on github if it helps to understand. I’m not much an expert in this subject, so I’m a little bit lost 
but I got the explanation from @BarryCarlyon and I will try it! 
Since I was using that 3rd party token generator I thought that my tokens would last the same amount of time and when I got ~15000s I was a bit confused!