Exercise 2 solution

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

    UTC = timezone.utc
    LON = ZoneInfo("Europe/London")

    def resolve(d, t, zone):
        z = ZoneInfo(zone)
        return datetime.combine(d, t, tzinfo=z).astimezone(UTC).astimezone(z)

    print("spring gap")
    for day in range(27, 31):
        r = resolve(date(2027, 3, day), time(1, 30), "Europe/London")
        print(date(2027, 3, day), "->", r.strftime("%H:%M %Z"), r.astimezone(UTC).strftime("%H:%M UTC"))

    print("autumn fold")
    d = datetime(2026, 10, 25, 1, 30, tzinfo=LON)
    for fold in (0, 1):
        x = d.replace(fold=fold)
        print("fold", fold, x.strftime("%H:%M %Z"), x.astimezone(UTC).strftime("%H:%M UTC"))
    print("same wall time, same offset?", d == d.replace(fold=1))
    print("hours apart:", (d.replace(fold=1).astimezone(UTC) - d.astimezone(UTC)))

Output (run, and exactly as shown):

    spring gap
    2027-03-27 -> 01:30 GMT 01:30 UTC
    2027-03-28 -> 02:30 BST 01:30 UTC
    2027-03-29 -> 01:30 BST 00:30 UTC
    2027-03-30 -> 01:30 BST 00:30 UTC
    autumn fold
    fold 0 01:30 BST 00:30 UTC
    fold 1 01:30 GMT 01:30 UTC
    same wall time, same offset? True
    hours apart: 1:00:00

Explanation: on 28 March 2027 the clocks in London jump from 01:00 to
02:00, so 01:30 does not exist that day. Converting through UTC and back
moves it forward by the length of the gap, to 02:30 BST. That policy keeps
the event on the same date and never drops it, but it lands at the same
UTC instant as the previous day's 01:30 GMT, 24 hours apart. On 25 October
2026 the clocks go back from 02:00 to 01:00, so 01:30 happens twice: fold=0
is the first time (BST, 00:30 UTC) and fold=1 the second (GMT, 01:30 UTC),
one hour apart. resolve() always picks the first, which is a policy you
should state in the toolkit's documentation. Note that the two aware values
compare equal (True): comparing two datetimes that share the same tzinfo
object ignores the offset and fold, so convert both to UTC before comparing
or subtracting, as the last line does.