The timestamps in /var/log/emerge should be readable. It's not that hard to do, and if you're worried about programs parsing it, date can use readable times easily, but I had to search the info pages to make it tell me when I last upgraded libxml2 (that was the second time - the first time I gave up before I could find it in the manpage). Reproducible: Always Steps to Reproduce: 1. 2. 3.
have a look at app-portage/genlop That does all you need. -> INVALID
The start of any merge set has a current local time. The current format is very easily machine parsable. I'll work on adding more human-friendly output though.