[manigandham]

It doesnt have to be rules, time zones have relative and changing references against UTC, for example daylight saving time which is 2x per year in the US.

If you schedule a meeting for 4pm Tuesday next week, but DST happens Friday this week, you still want the meeting on 4pm Tuesday next week. The local timezone and local timestamp is necessary for that.

[Arnavion]

If the software converts to UTC it should be using the expected local timezone on the target date (4pm Tuesday), not the current timezone. So this will only be a problem if the TZ tables for the future are not correct (which happens, as noted in sibling comments, but is not as widespread as every DST event like your comment seems to imply).

[manigandham]

> expected local timezone

I'm not sure what this means. Either way, to store an appointment for a certain user, you need the local timezone of that user and the local timestamp. UTC version of that timestamp is optional (maybe for easier calculations in the database), but you must have the local timestamp to convert correctly.

Local TZ => UTC is a formula that is not constant. You cannot store the output of this formula and expect to reverse and get back the original input precisely because of the changing formula.

[Arnavion]

>I'm not sure what this means.

The timezone that the user will experience on that date.

>Either way, to store an appointment for a certain user, you need the local timezone of that user and the local timestamp.

Correct. I did not disagree with this.

>Local TZ -> UTC is a formula that is not constant. You cannot store the output of this formula and expect to reverse and get back the original input precisely because of the changing formula.

Also correct. But as I said, your original comment implied this is a concern for all DST events which is not true. It is only a concern when the time was converted using some DST assumption and then that assumption is invalidated (eg the software decided to convert "4pm Tuesday local" to UTC assuming the timezone on Tuesday will be UTC+7, but the DST change happens to come early this year and in fact it's UTC+8 on Tuesday).

Regardless, I agree that taking this risk is not correct. Such a time should not be converted to UTC to begin with. Better to store the original local time.

[manigandham]

Yes, it is trivial extra work and storage for a timezone + timestamp (along with optional UTC) to get 100% reliable calculations.

Timezones, especially globally, have enough updates being made that they are not something worth the risk just to save a few bytes.

[openasocket]

That doesn't sound right. If you have a local timestamp with the date and the time and convert it to UTC, shouldn't the datetime library look up the rules for this timezone and convert it correctly? Isn't that the whole point of having timezones be more than offsets?

[manigandham]

What doesn't sound right? You might be saying the same thing. To store this appointment correctly, you store the timezone of the appointment (let's say EST) and the timestamp within that timezone (4pm). So the final version is 16:00 EST. You can store a UTC version as well, but you need the local timestamp and timezone first.

If you only store the timezone with a UTC timestamp, when you convert back after DST shifts you'll end up with 3pm or 5pm (depending on the shift), not the expected 4pm for the actual meeting. UTC in this case is not the anchor, the local timestamp the user set for the meeting is.

[openasocket]

No, I mean you make the appointment for December 3rd at 10:00am in Eastern Time (ET), and then you convert it to UTC to put into the database. Your local->UTC conversion library looks at the date and notices that this occurs during Daylight Savings Time for ET, and uses the correct offset when converting. Unless the rules about DST change between now and that meeting this is completely correct, you don't need to store the local timestamp at all.

[manigandham]

That is not reliable. Storing in UTC is optional, and can be used for easier comparisons, but you must store the local timezone and timestamp to be 100% reliable.

The local TZ => UTC formula may change for any reason, whether it's daylight saving time or any other random situation. It might be relatively stable in some locations (although it can change even within a global timezone like EST based on location) but there are constant updates being made. You can look at the IANA and Microsoft timezone database updates yourself. Why create risk when not necessary?

[0x0]

That was my original point. The rules about DST (or even just the base UTC offset) may change between now and the meeting unpredictably according to the whims of the local government, and this happens all the time.

[openasocket]

Oh yeah I totally get that, but my point was the only problem is from rule changes, not from regularly scheduled DST events.