How should an RFC 5545 duration of less than 24 hours act when it crosses a Daylight Saving Time (DST) transition?
For example, assume DST ends at 2:00AM on a particular day, and assume that an event starts at the second 1:10AM on that day. (The first 1:10AM is one real-world hour earlier in Daylight Time, the second 1:10AM is in Standard Time).
If that event has a -PT15M (15 mins before) duration reminder, then how many real-world minutes before the start of the event should that reminder pop up? Should it pop up 15 real-world minutes earlier at the first 1:55AM (in Daylight Time)? Or should it pop up 75 real-world minutes earlier at 12:55AM Daylight Time?
The spec seems to imply the latter behavior, but that behavior seems counter-intuitive to me. If I want a reminder 15 minutes beforehand, I really mean "15 minutes". However it is intuitive for longer reminders, e.g. -P1D should be 25 hours before the start of that event so that you get the reminder at the same local time on the previous day.
Anyway, how do most calendar apps deal with this unintuitive behavior? Do they ignore the spec and always treat reminders <24 hours as exact without adjusting for DST? Or am I understanding the spec incorrectly and it's the -P1D case that will be unintuitive with the one-day reminder showing up at a different local time as the event start time?
Obviously this is somewhat of a corner case because few meetings start in the middle of the night. But there are cases where this could happen, e.g. late-night social events. And if I promise to meet you at the bar in 2 hours, I probably don't mean 1 hours or 3 hours on some nights!
Here's what the RFC 5545 spec says:
In the case of discontinuities in the time scale, such as the change from standard time to daylight time and back, the computation of the exact duration requires the subtraction or addition of the change of duration of the discontinuity.
To clarify further, according to https://standards.calconnect.org/csd/cc-51003.pdf, RFC 5545 has a concept of nominal durations (in local clock time) and exact durations (a real-world stopwatch that ignores DST). The language above is about how a nominal persisted duration should be converted to an exact duration.
This question isn't specific to a particular calendar implementation, but I'm tagging Google Calendar and Outlook here because developers using those APIs probably have the most context about these issues.
Consolidation of answers and comments above and direct answers to questions posed:
First part:
"How should an RFC 5545 duration of less than 24 hours act when it crosses a Daylight Saving Time (DST) transition?"
Answer:
Since the duration is less than 24 hours, the exact number of hours, minutes, or seconds should be used for the duration (realtime). There is no confliction or ambiguity with durations < 24 hours.
Second part:
"If that event has a -PT15M (15 mins before) duration reminder, then how many real-world minutes before the start of the event should that reminder pop up? Should it pop up 15 real-world minutes earlier at the first 1:55AM (in Daylight Time)? Or should it pop up 75 real-world minutes earlier at 12:55AM Daylight Time?"
Answer:
15 real world minutes. The specs clearly say that anything less than one day is exact time. Any confusion or ambiguity around a daylight saving cutoff where a time is 'repeated' is avoided by adhering to the standard for referring a repeated clocktime during daylight savings. IE:
Third Part:
"how do most calendar apps deal with this unintuitive behavior? Do they ignore the spec and always treat reminders <24 hours as exact without adjusting for DST? Or am I understanding the spec incorrectly and it's the -P1D case that will be unintuitive with the one-day reminder showing up at a different local time as the event start time?"
Answer:
They don't ignore the spec. The specification deals with this as one would intuitively expect. A reminder of < 24 hours (PT24H) is exact. No need to adjust for DST.
Similarly a reminder of 24 hours (PT24H) before should occur exactly 24 hours before.
In the case of a one day (P1D) reminder, a 1 day reminder should occur at the same time the previous day be this 23,24,or 25 hours before. This is as one would intuitively expect. The actual number of hours will of course vary depending on the day's place in the calendar.
P1D is not the same as PT24H, as P1D will depend on the dates place in the calendar, whether DST changeover is happening that day, whereas PT24H is exact.
References: