Average time on page / session duration in Google analytics is 0

Why is average time on page and session duration 00:00:00 in Google analytics in some cases ?
Is it real time or is there a problem with measurement ?

Average time on page or session duration 00:00:00 in Google Analytics usually isn't accurate. You can see zero time because of measurement or calculation method.

You often see zero time on page or session duration because a visitor left your web without triggering Google Analytics to stop measuring the time. This occurs for example when user visits only one page. If "single page visitor" left the web by closing of browser or browsing tab, then average time on page or session duration is zero.

So if user spends on the web 5 minutes, but visits only one page, then it can happen that average time on page or session duration is 0. But if user visits multiple pages, you should see a real calculated time.