Another thing to consider is that transitions in offset are not always tied to transitions in DST, and you can (rarely) get transitions in DST that do not invol

Another thing to consider is that transitions in offset are not always tied to transitions in DST, and you can (rarely) get transitions in DST that do not involve an offset change. For example, Portugal changed their base zone to Western European Time from Central European Time during a scheduled DST transition. The result was that they switched over to being in "Summer Time" with no change in the clocks. There are also examples of non-DST related time zone changes of up to 24 hours (mostly from Pacific islands deciding to change what side of the international date line they are on), so in some zones, there are certain entire days that never happened (e.g. the entire day is an "imaginary day") and some that happened "twice".