Exercise 3 solution

    from datetime import datetime, timezone
    from zoneinfo import ZoneInfo, ZoneInfoNotFoundError

    def get_zone(name):
        try:
            return ZoneInfo(name)
        except ZoneInfoNotFoundError:
            return None

    for name in ["Europe/Paris", "BST", "Europe/Londn", "America/Chicago"]:
        zone = get_zone(name)
        print(name, "->", "unknown" if zone is None else zone.key)

    start = datetime(2026, 9, 3, 18, 0, tzinfo=ZoneInfo("Europe/Paris"))
    stored = start.astimezone(timezone.utc)
    print("stored", stored.isoformat())
    for name in ["Europe/Paris", "America/Chicago", "Asia/Tokyo"]:
        print(name, stored.astimezone(ZoneInfo(name)).isoformat())
    print(stored == start)

Output (run, and exactly as shown):

    Europe/Paris -> Europe/Paris
    BST -> unknown
    Europe/Londn -> unknown
    America/Chicago -> America/Chicago
    stored 2026-09-03T16:00:00+00:00
    Europe/Paris 2026-09-03T18:00:00+02:00
    America/Chicago 2026-09-03T11:00:00-05:00
    Asia/Tokyo 2026-09-04T01:00:00+09:00
    True

Explanation: abbreviations such as BST are not IANA keys and are rejected, as
are typos; catching ZoneInfoNotFoundError lets the program report a clear
message instead of crashing. The UTC value is the one to store. Every local
view is derived from it with astimezone, and the final True shows that the
stored value and the original local value are the same instant.