Had this issue come up at work recently. We were troubleshooting a database & SAN issue, and desperately needed timestamps added to dmesg. Finally found this helpful post on Arch Linux’s forums, much thanks to user “begray”.
To have timestamps added to dmesg entries, add the following parameter to your boot options:
Sadly, that didn’t help us in our situation (as we would have needed to reboot, nixing dmesg in our case), however it’s good to know for future treks. 🙂