With DST, there are actually 2 new concepts: ambiguous time (if the clock rolls back) and invalid time (if the clock jumps forward).

Java and Ruby work differently. Java would simply round the invalid time to the closest valid time IIRC. Ruby would accurately raise the InvalidTime exception. Same behaviors for an ambiguous time.

Chile is actually the country that will cause tricky issue in a system because they adjust DST at midnight... so there is one day a year where its midnight is considered invalid time. If we are building a system that depends on a day's boundary, then we will encounter this nightmarish issue where one of the days must start at 1am instead of midnight.

I really hope DST is going away soon.