Hacker News Reader

Hacker News

Comment by flabbergasted | Hacker News Reader

Parent Full thread

flabbergasted

As the very last line mentions, "use UTC" and avoid silly DST issues like this.

View on HN

My shop opens when the local clock tower says 9 am

I want a cronjob set up to change the “closed” sigh to say “open”.

How do I set that with UTC?

You store the time you want to see on the clock tower and the timezone where the clock tower is. Every hour you do the following pseudocode:

    if utc_now().to_timzone(my_timezone) >= "9AM":
        sign = "open"

I know you didn't mean this, but it was a problem I thought about: How does one make sure the local clock tower actually changes in cadence with DST? You can't, so I guess you could use open vz with a camera to have your open sign be based on what time is actually seen on the tower.

I know it's just an example, but I say in jest, don't use cron jobs to change your closed sign to open! If you can't make it to your shop, people will be waiting around all day for you to unlock the door. Very unhappy customers.

This is just duplicating the code already in cron. Of course you can make cron simpler by running your script every hour, and making the script more complex. You're just pushing the complexity to user-authored, non-reviewed, non-tested code. What is the advantage?

The first advantage is that you can dynamically check what your actual open hours are rather than having them statically configured (e.g. a web service to check for holidays). Open hours are never actually static. (Unless maybe cron does holidays?)

The second advantage is that you can check in multiple timezones especially if the server itself is in DST. Does cron itself let you set the timezone for each individual check? Or does it only run in one timezone? As far as I can tell, it only lets you use one timezone.

You can still run whatever business logic from cron to decide whether you took the day off or you have the resources to open. Offloading the timezone logic to cron still makes sense, since running tasks at the right time is exactly what it's for.

It's really about trading off something I understand fairly well (a script I wrote) and something I have questions about. Since it's not a particularly important problem and I'm more likely to make errors in cron, I'd rather have everything visible in a self-contained script.

For something important, I would look at it completely differently by testing it:

- What happens if I have a scheduled cronjob running every day at a certain time, but I want to override it for one specific day and have that job run later? (e.g. how is it accomplished so that I can maintain the default behavior while overriding and opening later on specific days)

- How can I make different commands run in different timezones? (as it stands now I find conflicting information about this)

- What happens if my machine lost power when the job needed to run? (it looks like it just never runs the script)

- Can I input a specific time and test if cron will set open for that specific time? (this doesn't seem possible)

some timezones go forwards/back 30 minutes with DST

and historically there have been even weirder cases

That’s a good point. It might make more sense to run it every minute or just have its own service if it’s that important. My use case is usually a user interacting with something that causes them to see something time specific.

This is a same problem in financial markets. Things like options contracts depend on a value observed say at 10am New York time on a particular date a decade in the future. If the daylight savings time schedule changes between now and then then the UTC time of the expiry of the options contract will change. The right reference isnt UTC or EST or EDT but "NY City local time" (or Tokyo or Warsaw or Mexico City or whatever is the relevant jurisidiction).

First of all, you’ll need a webcam pointed at the local clock tower…

Set the cron job to run every hour in UTC, and your application logic checks if its time to open in the configured time zone.

So the fix to using UTC for computers and crons that only care about the local time zone is to reimplement time checking?

If youre just gonna run it every hour, you might as well while(1){ check time; do stuff; sleep 1 hour;} now there's one less dependency?

It depends on that never dying.

You also depend on cron never having a bug, and you also depend on crons working as intended, unless you also have some extra system logging your cron jobs.

Unfortunately there’s more … interesting time zones than UTC+/-xx:00, and more interesting transitions than moving an hour.

For instance, Nepal uses UTC+05:45, and the time shifted by 15 minutes in 1986 (from UTC+05:30): https://www.timeanddate.com/time/zone/nepal/kathmandu

That said, I think running every 15 minutes handles all currently-observed time zones.

Open your shop at 0900 UTC

Would that it were so simple! For cronjobs, maybe it often is, but DST transitions are a mess to deal with in things like calendaring UIs and other places where you have to convert point-in-time values to the format consumed by normal people, and where your users can have different but legitimate expectations about how those transitions should be handled.

At one company I remember helping build a scheduling UI for a linear TV broadcast channel, where you plan out the broadcast day in 24h blocks. Worked great except twice a year when, due to DST, you have either a 23h day or a 25h day, and half a dozen ways various customers expected that to be handled. The de facto solution was to have happy customers 363 days a year. :-/

Hey, but thank goodness for https://en.wikipedia.org/wiki/Uniform_Time_Act or it would have been even worse!

A better answer is to use time+locale for things that are supposed to happen in the future and UTC for things that happened in the past.

The reason is that things that happened in the past will have a specific moment on the arrow of time at which they happened, and UTC is as good as anything else for putting a consistent label on the arrow of time.

But, for things that are going to happen in the future, you have no way of knowing if two "time labels" that right now point to the same moment on the arrow of time might have some change that causes them to point to different moments. In this case UTC removes the ambiguity about the timezone that the time is in, but it adds ambiguity about which moment in time you're really talking about.

Let's say you want to schedule something for 9pm local time (UTC-3). So you schedule it for midnight UTC. Then your timezone shifts to UTC-2 and your appointment moves to 10pm. That's likely not at all what you intended. It's also why you need to pick a "master" timezone when you're sending invites to multiple timezones and let the rest of the times be derived from the "single source of truth" time.

This heuristic obviously doesn't work. I build a factory in a strategy game. It is going to take 10h. The timezone shifts; should it take longer, shorter, the same? Obviously the same.

You still need to use your brain and careful UI design. Things that a user scheduled to happen at a specific local time need to store the relevant timezone, implicitly or explicitly.

And probably don't schedule anything for the last few seconds of a calendar year.

End of year is fine, it is around start of day / end of day when DST switches that screws up things

I was referring to leap seconds, which are applied at the end of the calendar in UTC, and are the only cases where UTC is not a continuous and predictable time scale. A scheduling system like cron still needs to have some mechanism to handle events that are scheduled in UTC, because particular UTC seconds can be skipped over. (And FYI, this can actually happen in other parts of the calendar year, not just the end.)

A leap second would be a change "less than 3 hours," so the job should still run according to the cited Debian man page.

Google's NTP servers smear leap seconds over an entire day, so UTC may appear continuous despite leap seconds.

https://developers.google.com/time/smear

Indeed. I only meant that simply switching to UTC doesn't actually reduce the necessity for the complex handling of scheduled events this article describes. It would just make these cases occur less frequently.

Reply on news.ycombinator.com