It turns out that the Pacific Time Zone is the only one where the bug caused a user visible difference because:

    1. Daylight saving time starting or ending changes the time zone offset by just one hour.
    2. The bug only has an effect when the difference in the number of hours goes from less than a day to at least a day, or vice versa (e.g. 23 to 24 or 24 to 23).

  The only hour of the day that satisfies those two conditions is 11:00pm, and the only time zone where daylight saving time starts and ends at 11:00pm is the Pacific Time Zone.