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This was an intentional design decision. We wanted to make sure all the temporal types could be serialize/deserializable, but as you mentioned, you couldn't implicitly go back to the object you started with as JSON.parse doesn't support that.

Instead the onus is on the developer to re-create the correct object they need on the other side. I don't believe this is problematic because if you know you're sending a Date, DateTime, MonthDay, YearMonth type from one side, then you know what type to rebuild from the ISO string on the other. Having it be automatic could be an issue if you receive unexpected values and are now dealing with the wrong types.

There is an example here in the docs of a reviver being used for Temporal.Instant https://tc39.es/proposal-temporal/docs/instant.html#toJSON

This is a real pain point and I run into the same tension in systems where data crosses serialization boundaries constantly. The prototype-stripping problem you're describing with JSON.parse/stringify is a specific case of a more general issue: rich domain objects don't survive wire transfer without a reconstitution step.

That said, I think the Temporal team made the right call here. Date-time logic is one of those domains where the "bag of data plus free functions" approach leads to subtle bugs because callers forget to pass the right context (calendar system, timezone) to the right function. Binding the operations to the object means the type system can enforce that a PlainDate never accidentally gets treated as a ZonedDateTime. date-fns is great but it can't give you that.

The serialization issue is solvable at the boundary. If you're using tRPC or similar, a thin transform layer that calls Temporal.Whatever.from() on the way in and .toString() on the way out is pretty minimal overhead. Same pattern people use with Decimal types or any value object that doesn't roundtrip through JSON natively. Annoying, sure, but the alternative is giving up the type safety that makes the API worth having in the first place.

I'm with you on this. I worked on a big Temporal project briefly and I was really turned off by how much of the codebase was just rote mapping properties from one layer to the next.

All Temporal objects are easily (de)serializable, though. `.toString` and `Temporal.from` work great.

> Especially in JavaScript where I often share a lot of code between the client and the server and therefore also transfer data between them, I like to strictly separate data from logic

Which makes me wonder how it'll look like when interfacing with WASM. Better than Date?

It should still be possible to continue using date-fns (or a similar lib) to suit your preference, right?

I would characterize it more as Joda likely informed Moment.js, which better informed TC39 because it was within the JavaScript ecosystem. As we discussed in plenary today when achieving consensus, every programming language that implements or revamps its date time primitives has the benefit of all the prior art that exists at that instant. TC39 always casts a wide net to canvas what other ecosystems do, but isn't beholden to follow in their footsteps and achieves consensus on what is best for JavaScript. So my view is this more represents what the committee believes is the most complete implementation of such an API that an assembled group of JavaScript experts could design over 9 years and finalize in 2026.

Yep, JavaScript got the bad version from Java too!

https://news.ycombinator.com/item?id=42816135

Deno has had it behind the `--untable-temporal` flag for quite a few Minor versions now and the latest Minor update (because of TC-39's Stage 4 acceptance and V8 itself also marking the API as Stable) removed the requirement for the flag and it is out of the box.

Node 26! Only a matter of time... :)

FWIW, I've been using it server-side via the js-temporal polyfill for some time, no issues.

Slower to implement new features, but still implementing them, just makes it the new Firefox. IE's larger problem was how popular it had been before it stopped implementing new features. It was like if Google got bored with Chrome and decided to stop all funding on it. People would be stuck on Chrome for years after that investment stopped because of all the Chrome-specific things built around it (Electron, Puppeteer, Selenium, etc and so forth).

Right now the world needs a lot more Safari and Firefox users complaining about Chrome-only sites and tools than it does people complaining about Safari "holding the web back". Safari's problems are temporary. Chrome is the new Emperor and IE wasn't bad because it stopped, it was bad because it stopped after being the Emperor for some time. People remember how bad the time was after the Empire crumbled, but it's how IE took so many other things down with it that it is easier to remember the interregnum after IE crumbled than to remember the heyday when "IE-only websites are good enough for business" sounded like a good idea and not a cautionary tale.

2026 A.D., still no support for native date pickers in mobile Safari.

And I have to support safari while dealing with all the problems that are mentioned in this article. Maybe there is a polyfill.

I usually am not too harsh on Safari on implementation of new features but this is a bummer, and reflects poorly on them

Right, browsers own it instead of websites needing to rebuild Moment.js bundles. Additionally, most browsers pass the ownership further to the user's OS as the IANA timezone database is a useful system-level service and best updated at the cadence of OS "required" updates.

Typically time zone data is updated in IANA's time zone database. That data would need to be updated in the implementation. In this case, the browser would need to update their time zone data.

If you give me your background I'll explain in longer terms but in short it's about making the intent clear and anyone who understands s modicum of PL theory understands why what's a constant is so and what's a function is so.

Storing in UTC is lossy. You've lost information about the event's original UTC offset, at the very least, and probably also its original time zone. Most backends today have good ways to round-trip offset information, and still compare dates easily (as if they were normalized to UTC). Some backends can even round-trip timezone information in addition to offsets.

It's easy not to feel that loss as a big deal, but captured offsets can be very helpful for exactly debugging things like "what time did this user think this was?" versus time zone math (and DST lookups) from UTC. It can help debug cases where the user's own machine had missed a DST jump or was briefly on a different calendar or was traveling.

But a lot of the biggest gains in Temporal are the "Plain" family for "wall clock times"/"wall calendar dates" and breaking them apart as very separate data types. Does a UTC timestamp of "2026-02-01 00:00:00Z" mean midnight specifically and exactly or where you trying to mark "2026-02-01" without a time or timezone. Similarly I've seen data like "0001-01-01 12:10:00Z" mean "12:10" on a clock without the date or timezone being meaningful, but Temporal has a PlainTime for that. You can convert a PlainDate + a PlainTime + a Time Zone to build a ZonedDateTime, but that becomes an explicit process that directly explains what you are trying to do, versus accidentally casting a `Date` intended to be just a wall-clock time and getting a garbage wall-clock date.

For recording instantaneous events, that's usually sufficient. It's often not enough for scheduling. You can always present UTC or any other zone relative to some other zone, but you need to know that zone. Maybe you're going to a conference in another region and you want to know the time of a talk in that zone because that's more important than your zone. You either need to couple the zone with the time itself, or you need to refer to it. There are good reasons either way. Having an atomic time+zone type is basically trading space for time. When its embedded, you can just use it, which can be better than assuming UTC and then looking up the zone based on, say, the location of the venue.

The only time you need local dates is for scheduling. Stuff like “Report KPIs for each shift. Shifts start at 8:00 local time.”, or “send this report every day at 10:00 local time”, or “this recurring meeting was created by user X while they were in TimeZone Z, make sure meetings follow DST”.

Outside of scheduling UTC is the way.

It does work quite well. Sometimes you need a time zone to go with it. It might not be common, but sometimes you need to know the local time in a particular zone, which is not necessarily where the user is. I work on software that works with local times in arbitrary time zones. We submit data in a schema over which we have no control, which must include such local times that may or may not be in the time zone of the server or the current client machine.

I have a scheduling system that allows users to specify recurring events. "Every Monday at 2pm." Which needs to be understood in the native timezone of that user and needs to be capable of being displayed in that timezone for all viewers or optionally in the native timezone of the viewing user.

Temporal is a blessing.

Same here, this is the way

why UTC and not epoch then?

More like a copy of Java’s JSR310, which in turn took many years to get right.

Yep. You can learn more about why we created this new blog here:

  https://bloomberg.github.io/js-blog/post/intro/

And if it seems like a surprise, you can blame me for not publicising this kind of content earlier given how long we've been working in this area. Thankfully Jon Kuperman and Thomas Chetwin (plus others) found the time and energy to put this platform together.

Bloomberg has a pretty large software engineering department, including a lot of offshore contractors. Similar to Walmart Labs that does cool stuff as well, despite being part of a retail chain (retail industry typically sees SWEs a cost, not asset).

What surprises you? Terminal UI is written in JS using Chromium. It’s not just plain Chromium, but it’s still funny that it’s pretty much same approach as universally (according to HN and Reddit) hated Electron.

https://youtu.be/uqehwCWKVVw?is=wBijGwdD2k2jIOu7