I think you may have missed the idea: usually dates (not datetimes) are the legal fiction "date" NOT "an instant." I.e. timezones are irrelevant.

Birth dates, contract dates, sale dates, billing dates, insurance coverage dates, etc.

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EDIT: UTC still plays a role -- as there is still the choice of calendar (though you'd be forgiven for assuming Gregorian) -- but it's an odd statement to decipher.

Ah! You're right, I totally missed that the commenter was griping about UTC dates as opposed to datetimes. I agree, a "UTC date" is not a clean concept. We can talk about Julian or Gregorian dates, but those are independent of time scales like UTC or TAI or UT1.

How are time zones irrelevant? The current date, at this very moment, depends on the time zone.

I think of date more like a date time simplified to day precision.

Still, I agree UTC date is unclear.

As you say, timezones are relevant if you need to covert an instant to a date, or vice versa.

But you can store, operate, and query on dates without the foggiest clue about instants or timezones.

But if you have a date, and a timezone...how do you do anything useful with those 2 things? If you add 8 hours to a date due to PST, what do you get?

UTC isn’t a time zone, its a specification for how many seconds are in a day. In UTC, there are 84000 seconds on most days, but the IERS may announce a “leap second” which makes some particular day either 84001 seconds or 83999 seconds (historically always the former).

That was already clear

> How are time zones irrelevant?

Nobody is going to adjust your birthdate when you move abroad.

If you move east, you’ll be able to drink a little earlier than if you move west.

Nobody is adjusting Christmas Eve against the time zone of Bethlehem.

There are many situations where a date is just a number in a calendar and not a specific time on planet earth.

> Nobody is going to adjust your birthdate when you move abroad.

But that’s just a convention, right? The day you are born is still dependent on the time zone. If you are born in the US at 11pm EST then someone born at that same moment in the UK has a different birthday.

Dates have boundaries. These boundaries are dependent on time zone. We can talk about dates irrespective of time zone but day periods cannot be understood without reference to time zone.

You are mixing two things up into one conversation. You are adding time into the conversation, in which case yes you need timezones, but if you don't add time into the conversation and just have dates, then you don't have timezones.

> But that’s just a convention, right?

Yes, "just."

Dates are "just" a convention.

A bit of an aside: by 2035, UTC will be a static offset from TAI, with the addition of leap seconds being phased out

That will be a wonderful change. Unfortunately we will still have to deal with leap seconds retrospectively, but it is certainly a step forward, and it will be nice to have a static list of historical leap seconds rather than all computers everywhere polling IERS every month or two.

> Lot of confusion here. UTC is a time standard, not a particular time zone. An instant written down in, say, the Pacific Standard Time Zone can be a UTC-scaled time.

UTC is very much a timezone, that’s why it has a timezone designator (Z).

If you record a future PST date as UTC and PST changes, your recorded date is now wrong.

> So for a date to be “in UTC” really just means it uses the leap seconds published by IERS. This article says “integers in UTC” which is a little ambiguous, but probably means “integer UTC seconds since the Unix Epoch.”

And that’s guaranteed to fuck up for future local events.

> If you record a future PST date as UTC and PST changes, your recorded date is now wrong.

Damn. That's a very good point. From now on, I'll be always recording also timezone, especially when it comes to future dates!

Any timezone you want, as long as it's UTC. (If above scares you, think about how to record an event that should happen 5th of November 2023 1:30 AM PST.)

> UTC is very much a timezone, that’s why it has a timezone designator (Z).

This is just not correct. The "Z" is a historical maritime designation for the zero-point timezone, GMT, and predates UTC by decades.

UTC is defined by the International Telecommunications Union in recommendation TF.460-6 (pdf link: https://www.itu.int/dms_pubrec/itu-r/rec/tf/R-REC-TF.460-6-2...). It's brief and you won't find any mention of time zones.

The symbolism "UTC+0" or the "Z" timezone come from ISO-8601, which is a specification for how to represent times in strings. You can find that specification here: https://web.archive.org/web/20171019211402/https://www.loc.g...

See section 2.1.12 Note 3:

> UTC is not a time zone, it is a standard. UTC is also not GMT (Greenwich Mean Time), rather, UTC has replaced GMT. UTC is more precise; the term 'GMT' is ambiguous.

That document goes to great lengths to keep this distinction between time scales and time formats. They're different, and conflating them will get you very confused. When you describe a time in PST, you are almost certainly using UTC.