The question is whether the REST Api should return an ISO8601 formatted string in UTC timezone, or the unix time (number of milliseconds since 1970-01-01).
There is quite some controversy about it.
In my opinion, the ISO8601 format (always UTC) is better. The reason is the better readability. But elm folks are on the other side:
One can convert from an ISO8601 date-time string in UTC time into the epoch millis and vice versa. So it is the same to me. There is no less information in a ISO8601 string than in the epoch millis.
To avoid confusion, all date/time values should use the same encoding.
I go with the epoch time. Every timestamp/date-time values is transfered as Unix timestamp.