Comment by pseidemann
9 hours ago
I'm referring to a "datetime", which should be a data type which stores a date, e.g. "2026-09-28", and a time, e.g. "16:58:18". The unix epoch is not relevant here, unless you _want_ to store a UTC timestamp. You could store a UTC timestamp also as ISO 8601, which is not a number. But this is not related to the timezone-less datetime I'm talking about. To make my points more clear, just imagine the datetime is stored as a string "2026-09-28 16:58:18" with no implied timezone whatsoever.
But there is zero use case difference between these two things you mentioned:
> if a point in time should be sticky to a calendar store a datetime _without_ a timezone
> If you want a point in time which will not "physically" change, store a datetime _with_ a timezone
Both of these things are exactly identical. You are still storing an exact point in time in both cases. The only thing about a calendar use case is the presentation layer.
> You are still storing an exact point in time in both cases.
No this is not correct. You store two different intents. Suppose you want a reminder in your calendar every day at 15:00 for the next 7 days. It should not change if DST changes in the middle of the seven days. It should not change if legislation of my timezone changes. How do you encode this? A UTC timestamp for each of the seven days will change the hour if the DST changes or the timezone changes. So what you can do is store a datetime _without_ a timezone, exactly reflecting what the user entered when creating the calendar entry, which is "2026-09-29 15:00:00", "2026-09-30 15:00:00", etc. This is the intent wanted by the user, which is a "sticky" time in their calendar. These are not exact points in time, since timezones can and will change (DST), so the "physical" instant of 15:00 can be a different "actual" (as in, the sun has a different position, earth rotation is different) time when comparing the days. Instead, these are exact times in 7 days only in a (specific) calendar.
That system you described is quite rare - it would have to poll N databases of N possible events constantly to see if it should give you a reminder. Which is why none of the common calendar systems do that.
They just create events, which have a time zone and represent and exact point in time. And they don't change the event or when you get notified when you change time zones.
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