Hello bouhlal !
I guess it's no necessary because:
When time is displayed it is wrapped in <time> HTML5 tag, then it is formatted according to the current settings using JS. As the result current user timezone (which is set in the OS) is used - no need to set timezone in UNA. Since JS is used it updates relative time in real time, so 2 minutes ago changes to 3 minutes ago and so on. The use of <time> tag allows search engines to know the exact date and time.