There are several dates shown to moderators on 'Mod dashboard' tab of a user's profile. For most dates on the Stack Exchange sites, if you hover your mouse over them, they will show the exact date, down to the second, as a tooltip.
However, this tooltip does not appear for the 'Joined network:' and 'Joined site:' fields. So, those might say something like '3 days ago' and there's no way to find out exactly when the user joined the site by hovering over.
Please fix this bug, so that those two dates behave like other dates and hovering will display the tooltip.