Should plus be commutative though? What do you think should be the laws of algebra involving all of the date-times, intervals and periods?

Currently the API follows the Java object model where the date/time always comes first when a period/duration is added/subtracted. In Java this happens naturally due to the placement of methods. In Clojure we can do as we like. However, date-times and durations are different types - it's not obvious that operations including both should be commutative. Maybe the plus/minus naming could be changed to something like add-duration or extend to clear up the behaviour (although I quite like plus), what do you think?

To be honest, I didn't really give it much thought so I cannot answer your questions properly. It's very well possible that the current state of affairs is the best one. It was just not really intuitive for me (at first, I even started searching in a completely different direction), and I didn't find any previous discussions on the matter.
If all is fine as it is, the issue can be closed and left just as a documentation for someone with a similar concern.