Earlier quoted context omitted.
Sometimes it's best to store a scheduled events as "Event localtime" and "Timezone" (where timezone is a named description - e.g. "Europe/Madrid" - rather than an offset - e.g. "+1:00"). This allows the record to stay consistent, even if there are changes to the local time rules - e.g. leap seconds, daylight savings, timezone offset. Imagine a tech-camp had been planned in Cairo, Egypt, to start on 9am on July 10, 20…
Yup, I'm aware of this and should have mentioned it in my comment. Thanks for the follow up. As an aside, how often do the tz databases for each language get released? Are they usually responsive to notices 3 days out? Edit: I went looking into the pytz release for the Cairo example from parent. Olson Timezone Database: Release 2016f - 2016-07-05 16:26:51 +0200 https://github.com/stub42/pytz/commit/03a4e9b31dd90f3dac…
I've read that the explanation for this temporary suspension of daylight-savings is Ramadan [2], and Ramadan is dependent on the observed sighting of the new moon - so you can't necessarily predict the date in advance.
I ended up coming across that after looking for an explanation for something bizarre I experienced on a trip to Morocco in March 2016…with my iPhone set to use "Marrakesh, Morocco", the time on the phone displayed correctly, but the time on my sync'd Apple watch was an hour out. I think I ended up manually setting it to Paris time to get the correct time, but never did get an explanation for the difference.
So even across two devices from the same manufacturer, theoretically sharing the same date-time information, they can be inconsistent.
Conclusion: time is hard!
[1] https://www.timeanddate.com/time/change/morocco/tanger?year=...
[2] http://codeofmatt.com/2016/04/23/on-the-timing-of-time-zone-...