[msandin]

Unless you do recurrent scheduling. In that case it's important to understand that you schedule in a particular timezone. Both because the relationship between local time an UTC changes throughout the year with DST changes and because evaluating an expression like "Thursdays at 03:00" might evaluate differently if you first move the time to UTC (say 22:00) and then check if it's a Thursday (no, it's only Wednesday still, but in 24 hours it'll be time).

[pavel_lishin]

Doesn't this become completely unglued once you've got people in different time zones, with different DST settings, using the same calendar?

If I have a meeting every day at 10am local time with someone in Arizona, their meeting suddenly jumps by an hour when my time changes. (Similarly, if it's my Arizonian colleagues who set the meeting to be at 10am daily, I'm suddenly meeting them at 11am my time once spring hits.)

[msandin]

Indeed, but this is representative of a very real change in the relationship between your clocks. The only way to keep this non-surprising is to make the time zone the schedule runs in explicit to the user. If somebody in a different zone edits the schedule it has to remain in the original time zone unless they explicitly change it. You do all your scheduling calculations in the schedule time zone and then convert it to an instant in time, likely expressed as a UTC time, when you need to calculate how long your computer should wait until the next event.

(Date)Times stored in the schedule (start times, end times, trigger times, clock times) are not instants in UTC, they are time specification in a calendar system which can only be interpreted as specific instants in the context of a particular time zone. Which is stored explicitly and separately from the calendar date-times.

This crucial separation of a time specifications in a calendar system from instants in time is one thing which many date/time libraries get wrong and which Noda-/JodaTime gets right.

[andrewl-hn]

Yes, and that's exactly what happens in practice in every calendar system out there. Usually event creator's timezone takes precedence, and for them the calendar stays the same, while other parties have their events suddenly jump by an hour.

[busterarm]

How you store and how you represent time are different but related problems.

As for how you should store times, yes you should always store time as UTC. This is a different problem.

[Retric]

It is reasonable to represent reoccurring events outside of UTC. If your user wants an alarm is at 7:30 AM MTWTF then you can store it just like that. If they travel to another time zone and they still get a 7:30 AM wake up call that's generally desired behavior. And yes there are a huge number of edge cases.

It's specific times you want UTC, if I setup a meeting then I want someone in another time zone to call in at the same time.

[busterarm]

I'm sorry, but if you have an application dealing with time and you want to even think of defining a behavior independent of time zone, you should still be storing values in UTC.

Your example may work fine on a server that you control, but let's say your use case is a phone alarm program. You can't guarantee that your application will know when it's 7:30AM in the local time, but you can guarantee what the time is in UTC and not need network or the device's help to do it.

So now the time format you store depends on the architecture of your platform? What if you have to send this data between platforms now? Am I going to have to convert back and forth for each message? No thanks. I'll just store everything as UTC and stay sane.

[msandin]

Please make the distinction that _instants in time_ should be stored in the UTC but time specifications for scheduling purposes (10:30, mondays 14:50, 2017-05-17T13:40) are not instants in time, they are specification of time to be interpreted in a specific time zone and should be stored as such.

In a very real example there was once a product which let you schedule events by specifying a start date and a recurrence interval, say "daily". The software converted the start time to UTC in the zone of the schedule creator and stored that. But the conversion was done at the creation time which means it was done with a particular offset (say -5 hours) and when the relationship between the creators time zone and UTC changed as a result of DST changes the schedule ended up one hour of from what the creator intended. And the information about which timezone the thing had originally been scheduled in was lost and could not be retrieved.

[busterarm]

What was this like...1990?

This is 2017. Are you not putting all of your transactions in an ordered table in a database with creation timestamps? That information should not be lost and should be easy to figure out, in UTC. Figuring out the conversion of some other time/date is a solved problem.

There should never be a question of when some work happened if you're following sane practices.

I do huge ETL projects that live or die on the accuracy of our timing data. Give me date(time)s in UTC and I'm happy. Anything else? Pain.

[msandin]

Of course we knew who did what and when, that was not the problem and I never said it was. But the start date+time they entered for recurring series was converted to UTC on the client before it was sent to the server. So when it was decided that what they really wanted wasn't to run it "every 24 hours after the start date+time" but to run it on the exact time they entered every day, starting on the day they entered, the information about the exact time they had entered was gone.", all we had was that time converted to UTC. This meant we could not deduce the original time zone in which it was entered and thus could not deduce which DST schedule it should follow to keep it running at the same time every day.

[msandin]

The answer to the question "when did some work happen" is an instant and I specifically said that instants should be stored in UTC. I'm talking about storing recurrent schedules, i.e. information about when an (potentially infinite) series of future events should take place. And that information is not a series of instants and can't be stored as UTC timestamps because it's potentially infinite, subject to future reconfiguration, subject to future timezone offset changes, and must be preserved in a format that allows it to be viewed and edited (Thursdays odd weeks at 10:00 is not easy to glean from a series of pre-calculated numbers).

[r00fus]

Yes, agreed - my iPhone alarm at 6AM for my workout should trigger whether I'm in Florida or California, and does.

[r00fus]

Disagree. DST is actually part of the time signature.

You are either in DST or not - that's why we have PST and PDT - one is a daylight time and reflects a different offset from standard.

Most people are in the same timezone but when not, it's the responsibility of the attendee to know that it's shifted - and the calendaring system should know to update it accordingly.