[mastermedo]

I dislike the "Plain" prefix to the temporal objects, e.g. PlainDateTime. The prefix says nothing about the behavior of the class. Plain as opposed to what; ZonedDateTime. I would prefer "Local" over "Plain"; LocalDateTime.

[exyi]

Local would imply the date is in the current machine timezone, while PlainDateTime is zoneless. It may be in the server timezone, or anything else. The main difference is that it does not make sense to convert it to Instant or ZonedDateTime without specifying the timezone or offset

[agos]

you have to consider the existence of PlainDate which is a Date without both a Time and a TimeZone