Datetime question

This has likely been asked before, humble apologies. What happens to the datetimes when the time changes in the fall (goes back one hour). In particular, there will be an hour in which datetimes are not unique. If the change happens at 2:00 am, then the datetime that would have been 2:00:01 is now 1:00:01 on the same date (which already occurred an hour ago). For example, suppose we have a simple text file with rows of events, each with a starting and ending datetime. If our objective is to find the duration of the event (the interval), how do we make sure that the interval is not affected by the ambiguity in datetimes? Cheers