The REST API v2 date-time strings should conform to RFC 3339 so that in an OpenAPI description of the JSON objects the various date-time strings can be annotated with

type: string
format: date-time

When the date-time strings do not strictly conform to RFC 3339 and the format: date-time annotation is used then the generated Java code (and presumably other languages with strict typechecking) will fail to validate the JSON being consumed.

If the format:date-time is not used then one loses a good deal of power and control with the generated API client code because then the JSON strings are cast to primitive String objects instead of other objects with more capabilities to manipulate the date-times.