Exercise 1 solution

    from datetime import datetime, timezone
    from zoneinfo import ZoneInfo

    ny = ZoneInfo("America/New_York")
    for day in range(7, 11):
        wall = datetime(2026, 3, day, 2, 30, tzinfo=ny)
        utc = wall.astimezone(timezone.utc)
        real = utc.astimezone(ny)
        flag = "" if real.replace(tzinfo=None) == wall.replace(tzinfo=None) else "  <-- does not exist"
        print(f"{wall:%d %b %H:%M} -> UTC {utc:%d %H:%M} -> back {real:%d %b %H:%M %Z}{flag}")

Output (run, and exactly as shown):

    07 Mar 02:30 -> UTC 07 07:30 -> back 07 Mar 02:30 EST
    08 Mar 02:30 -> UTC 08 07:30 -> back 08 Mar 03:30 EDT  <-- does not exist
    09 Mar 02:30 -> UTC 09 06:30 -> back 09 Mar 02:30 EDT
    10 Mar 02:30 -> UTC 10 06:30 -> back 10 Mar 02:30 EDT

Explanation: only 8 March fails the round trip. The clocks jump from 02:00
straight to 03:00 that morning, so 02:30 never appears on any clock. Python
does not raise an error: it silently picks an offset and builds an object, and
the round trip through UTC reveals the true reading (03:30). Note that the UTC
times are one hour apart on 7 and 8 March but the wall times are 24 hours apart:
the offset changed from -5 to -4 between those days.