Something like “9AM-5PM EST” can’t be stored in a datetime object even with a timezone slot! Datetimes with a timezone represent specific points in time, not vague “5PM” like concepts.

I feel like every time a discussion on timezones comes up, people compete to come up with different situations where "see, your perfect system can't work, we should give up on timezones entirely".

Here, I'm not even sure what your point is. A datetime object cannot capture a range, no, just like a number can't capture a range of values. But through the magic of having two of them, we can get easily create a data structure containing two datetimes to represent a range. 5pm is not a vague concept, it is the time 5pm, the hour that typically comes after 4pm and before 6pm. If your point is that you can't store only a time in a datetime object, that's true, that's why the standard library also provides the `time` object which represents a time.

Where things might get a bit more complex is if you want to store the time of a recurring event that should occur at the same time on multiple days for a given timezone. In this situation, you can typically use a naive time object to represent a wall clock time, plus the timezone that the user has requested. This way you still have all the raw information to decide when the event will happen in the future. (Note however, that the same time can occur multiple times on the same day, for example during daylight savings changes.)

"every day, from 9AM to 5PM EST" is a thing that can be represented through calendar RRULEs. Usually libraries exist to help with this (Python has one, conveniently called rrule!). This is a thing to describe recurring events, has a spec and everything. So there is a thing.

And you're totally right about `time` existing, it slipped my mind that time has a tzinfo slot. I do think you should still take care and I would avoid having any naive time objects just floating around beyond the serialization layer though.

Yeah, the stock market opens at what time EST? Including daylight saving time, etc. I agree that any time time goes to a serialized format over a socket or to a database or file it should always be UTC.

UTC (or Unix timestamp, or any other non-localized date-time format) doesn't work for future localized events. It's fundamentally impossible to say what will be the UTC timestamp which corresponds to 9AM in some particular location in the future: the timezone of that location could arbitrarily change between now and then. Countries and smaller areas often experiment with such changes, wars happen and conquerors/liberators change the time etc.

So, when possible, it's best to store (future) human events in a localized datetime string. For things like physical events, the opposite is true of course - you can't say what will be the local date and time when the Sun will hit high noon over Athens 10 years from now, but you can certainly calculate the UTC timestamp for that.

Ignoring holidays/weekends: always 9:30AM or (0930) EST. Same with EDT: 9:30AM.

The key here is that the timezone info includes the daylight/standard designation. Or, in the ISO format -4:00 or -5:00 from UTC.

I think you meant to put ET (Eastern Time), which is still 9:30AM, but without a date associated with that time, there's no way to convert that to UTC or other timezones that don't have the same daylight saving time schedule.

Why can't opening hours be stored in UTC?

A store’s opening hours will probably remain 9am-5pm regardless of any time zone changes around it. If you run a bunch of stores across time zones you need to know where each store is and whether it’s summer or winter to work backwards to find out 9-5 in UTC.

The coffee shop in my neighborhood is open from 6am to 2pm Pacific. We observe daylight saving time. The shop hours do not change. So, what UTC value do you store?

I guess it would change based on local time changes.

Seems like the ideal way to store them to me!

Then you would be wrong, because they open at 9am local time - if the government changes local time with 2 days notice then opening time changes.

Have you thought about this? How do you store shop hours in UTC for shops in timezones with DST?

Every heard of daylight savings time (DST)?

This is only sometimes true. Cases like opening hours and daily batch processing rely on local time. Persisting the values in UTC mean best case you are wrong half of the year and worst case all of it.

Just because you're storing the time in UTC doesnt mean you can't also store the timezone, and lots of other related data too.

6am in my time zone is two different times in UTC. Right now it is 14:00 UTC. But when DST starts it will be 13:00. So what UTC value do you insert into your database for the opening time of the coffee shop? And how do you convert?

Yes, you can store additional information to decipher the meaning of a UTC time but why? Storing locale is good because sometimes you need to know where something is, such as when converting the local time to another zone. But you should avoid creating dependencies between these values because it makes using them individually harder.

The coffee shop opens at 6am. That is an unambiguous fact. The coffee shop is in Seattle, WA. That is another unambiguous fact.

If the time is in UTC then you need to know the date the time was set or store a dtc flag and then build some logic to convert. You can just feed local time+locale into any datetime library and get whatever you need.

UTC is great for a lot of things. Such as recording when something happened. But local time has a place as well.