Change R to UTC

I want to change Sys.timezone in R to UTC. Actually I have it on "Europe/Madrid".

To change it, I have tried changing Debian distribution with

 sudo vim /etc/timezone

And there change Europe/Madrid to UTC.

After that I have restart Debian and check Sys.timezone() in R, but still apears Europe/Madrid.

I also have tried with

 Sys.setenv(TZ='GMT')

But it does not work

How can I solve this issue?

Thanks!