What do you mean exactly by UT ?

In the code you're referring time as 👍 

"Struct containing date and time to compute the position for, in UT"

What type of UT are you assuming ? 
https://en.wikipedia.org/wiki/Universal_Time#:~:text=Universal%20Time%20(UT)%20is%20a,up%20to%20a%20few%20seconds.&text=UTC%20is%20based%20on%20International,within%200.9%20second%20of%20UT1.

Or maybe i'm misunderstanding something... 