Any way in Github.com to see the exact time for a commit or release

If you move your mouse over the xx minutes|hours\... ago label, a tooltip will be displayed showing the UTC time the commit was created at.

Keep in mind that Git history can be rewritten (see git commit --amend or git filter-branch commands for instance) and that this displayed time shouldn't be considered as solid proof.

Well I'm jumping into this WAAAYY late, but I wanted to add a note about this particular use case.

nulltoken is absolutely right about the possibility of the times being modified. To circumvent this, I would make use of tags. Slap together a simple script that would pull the branch at the cutoff point, tag it (using an annotated tag), and then push the tag up to github. This gives you a snapshot of the branch at that particular time, stamped the time that the tag was cut AND the name of the tagger. That last part is particularly useful for students who think they are crafty.

https://git-scm.com/book/en/v2/Git-Basics-Tagging

Cheers!