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However, no correctly formatted ISO8601 formatted date is accepted.
Any attempt to use an ISO8601 formatted date receives the following error message: only date-time value with 'yyyy-MM-dd HH:mm:ss z' format is allowed. Atleast that clues me in that spaces are preferred over a "T" between the date and time.
However, using "Z" to indicate UTC timezone is not accepted.
Basically, the only date format accepted is a PST/PDT timezone offsetted date. A UTC date is not accepted. Not only that, the time zone portion must be either "PST" or "PDT", not "-7", "-8", etc. This does not adhere to the ISO8601 format which does not support timezone names, only offset values, according to the very docs your documentation links to.
Please update your REST API to support ISO8601 dates.
Properly document the format of your accepted dates.