Exercise 1 solution

    from datetime import datetime, date, timezone
    from zoneinfo import ZoneInfo
    import json

    def encode(o):
        if isinstance(o, datetime):
            if o.tzinfo is None:
                raise ValueError("naive datetime: attach a zone first")
            return o.astimezone(timezone.utc).isoformat(timespec="seconds").replace("+00:00", "Z")
        if isinstance(o, date):
            return o.isoformat()
        raise TypeError(f"cannot encode {type(o).__name__}")

    def dumps(obj):
        return json.dumps(obj, default=encode, sort_keys=True)

    def loads(text, instants=(), days=()):
        data = json.loads(text)
        for key in instants:
            data[key] = datetime.fromisoformat(data[key])
        for key in days:
            data[key] = date.fromisoformat(data[key])
        return data

    order = {
        "id": 42,
        "placed": datetime(2026, 11, 2, 9, 30, tzinfo=ZoneInfo("America/New_York")),
        "ships_on": date(2026, 11, 5),
    }
    text = dumps(order)
    print(text)
    back = loads(text, instants=("placed",), days=("ships_on",))
    print(back["placed"], back["ships_on"])
    print(back["placed"] == order["placed"])
    print(back["placed"].utcoffset(), order["placed"].utcoffset())
    try:
        dumps({"placed": datetime(2026, 11, 2, 9, 30)})
    except ValueError as e:
        print("ValueError:", e)
    try:
        dumps({"x": {1, 2}})
    except TypeError as e:
        print("TypeError:", e)

Output (run, and exactly as shown):

    {"id": 42, "placed": "2026-11-02T14:30:00Z", "ships_on": "2026-11-05"}
    2026-11-02 14:30:00+00:00 2026-11-05
    True
    0:00:00 -1 day, 19:00:00
    ValueError: naive datetime: attach a zone first
    TypeError: cannot encode set

Explanation: 09:30 in New York on 2 November 2026 is UTC-5 (daylight saving
ended on 1 November), so the instant is 14:30 UTC. The round-tripped value
equals the original, because aware datetimes compare by instant, but it no
longer carries the New York offset: it comes back as UTC (the last line shows
an offset of 0 against -5 hours, which Python prints as -1 day, 19:00:00).
If the original local time must be shown, store the zone name separately and
convert for display. The encoder tests datetime before date because datetime
is a subclass of date, and it rejects naive values at the boundary rather
than guessing a zone.