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by openasocket 3304 days ago
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?
1 comments
manigandham 3304 days ago
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.

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openasocket 3304 days ago
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.
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manigandham 3304 days ago
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?

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0x0 3304 days ago
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.
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openasocket 3304 days ago
Oh yeah I totally get that, but my point was the only problem is from rule changes, not from regularly scheduled DST events.
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