Why is time taken in hibernation included in system uptime?

Solution 1:

shouldn't uptime be resetting to zero if the 'the system is fully powered off'

• suspend/hibernate are not considered "off". The last "wake up time" is stored in /var/log/pm-suspend.log.

or pausing because the system is hibernated

• now that would be odd. How would one then know what the actual uptime is?

or is this a bug?

Uptime is the time since last (re)boot. Basically the system stores at boot time what the time is and "now" -/- that time is the uptime. So in that regard: no, not a bug. The command predates notebooks and hibernate/suspend so maybe if you create a feature request someone decides it is a good feature to have. It probably is a very easy addition:just update the time stored on disk. And I can not imagine it is a lot more than adding a call to the hibernation/suspend script. But it will also be difficult to get it happen :)