Example of what it looks like in Piwik 2.15.0:

I think ideally it would always say something like 1 hour ago, 10 minutes ago, 1 month 10 days ago, 1 year ago. On hover, if needed, it could say Jan 01 2015 00:04:01 in the website's timezone UTC+8, which is ... in your timezone (and ideally we would also show it in users current timezone when it is different).

Showing a date here is not really helpful, especially when UTC is different than the websites timezone which might be again different of the users timezone. One never knows which one is used and even when you know it, you have to do the math to convert it to your timezone etc.

With JavaScript we could update the time when hovering... eg by having a timestamp in the HTML attribute or so. This could be an angular directive. Whenever hovering it, it would update the time and say eg "5 minutes ago", then 5 minutes later when hovering again say "10 minutes ago"

There is an issue with instances that have the default timezones set to countries way ahead of GMT, like Australia. It seems like reports generated before UTC 00:00 will show "report generated on {{date}}", which some users (in EU or US) will see a report from yesterdays date. If the report is generated after UTC00:00 then "Report generated {{hours}} ago" which is a much clearer option.