Exercise 3 solution

    from datetime import datetime, timezone
    from zoneinfo import ZoneInfo

    # Part A
    def parse_instant(text):
        try:
            dt = datetime.fromisoformat(text)
        except ValueError:
            raise ValueError("not an ISO 8601 date-time") from None
        if dt.utcoffset() is None:
            raise ValueError("missing offset")
        return dt.astimezone(timezone.utc)

    for text in ("2026-11-02T12:00:00Z", "2026-11-02T13:00:00+01:00",
                 "2026-11-02T12:00:00", "2026-11-02", "02/11/2026 12:00",
                 "2026-13-02T12:00:00Z"):
        try:
            print(f"{text:27} -> {parse_instant(text).isoformat()}")
        except ValueError as e:
            print(f"{text:27} -> rejected: {e}")

    # Part B
    record = {"local": "2027-03-28 09:00", "zone": "Europe/London"}

    def instant_now(rec):
        local = datetime.fromisoformat(rec["local"]).replace(tzinfo=ZoneInfo(rec["zone"]))
        return local.astimezone(timezone.utc)

    frozen = instant_now(record)
    print("Instant frozen at booking time:", frozen.isoformat())
    print("Recomputed from local + zone  :", instant_now(record).isoformat())

    # Pretend the zone's rules changed, so that it stays on UTC+0 all year.
    changed = datetime.fromisoformat(record["local"]).replace(tzinfo=timezone.utc)
    print("Recomputed after a rule change:", changed.isoformat())
    print("Frozen instant shown locally  :", frozen.astimezone(timezone.utc).strftime("%H:%M"))
    print("Meeting should still be at    :", record["local"][-5:])

Output (run, and exactly as shown):

    2026-11-02T12:00:00Z        -> 2026-11-02T12:00:00+00:00
    2026-11-02T13:00:00+01:00   -> 2026-11-02T12:00:00+00:00
    2026-11-02T12:00:00         -> rejected: missing offset
    2026-11-02                  -> rejected: missing offset
    02/11/2026 12:00            -> rejected: not an ISO 8601 date-time
    2026-13-02T12:00:00Z        -> rejected: not an ISO 8601 date-time
    Instant frozen at booking time: 2027-03-28T08:00:00+00:00
    Recomputed from local + zone  : 2027-03-28T08:00:00+00:00
    Recomputed after a rule change: 2027-03-28T09:00:00+00:00
    Frozen instant shown locally  : 08:00
    Meeting should still be at    : 09:00

Explanation: Part A accepts only strings that carry an offset (Z or +hh:mm),
converts them to UTC, and gives a distinct reason for each rejection. A bare
date parses successfully but has no offset, so it is rejected for that
reason; the day/month/year string and the month 13 fail to parse at all. Part
B: on 28 March 2027 London has just moved to summer time (UTC+1), so 09:00
local is 08:00 UTC. If the zone's rules changed to stay on UTC+0, the meeting
should still be at 09:00 local, which is now 09:00 UTC. The instant frozen at
booking time (08:00 UTC) would show as 08:00 local and be an hour wrong, while
the stored local time plus zone name gives the right answer simply by being
converted again with the updated rules.