javascript datetime utc ecmascript-temporal

How to create instance of `Temporal.Instant` at specific date and time?

I’m struggling with JavaScript’s proposed new Temporal API. What I am trying to do should be straight-forward, yet I fail to find a convincing solution. I must be missing something.

The task is as follows: instantiate an object representation of an UTC datetime from variables for year, month, day, hour and minute.

My thinking is as follows:

we are talking UTC so I need a Temporal.Instant;
new Temporal.Instant() requires the timestamp in nanoseconds so that doesn’t work;
Temporal.Instant.from() requires a ISO datetime string, which would require me to generate a properly formatted piece of text from the five variables I have — this is possible but a bit of a hack and kinda defeating the purpose of using a datetime library;
Temporal.PlainDateTime.from() has the right design, as it accepts an object like { year, month, day, hour, minute };
so then all we need to do is creating an Instant from this PlainDateTime. This does not seem to be possible though? Other than through — once again — a datetime string or a timestamp in ns…?

This is silly! The use case here is super basic, and yet it’s not obvious (to me) at all how to address it.

I was expecting to be able to simply do something like: Temporal.Instant.from({ year, month, day, hour, minute });

Now the best I can come up with is: Temporal.Instant.from(year + '-' + String(month).padStart(2, '0') + '-' + String(day).padStart(2, '0') + 'T' + String(hour).padStart(2, '0') + ':' + String(minute).padStart(2, '0') + 'Z'); // 😱

Please tell me I’m bigtime overlooking something.

Solution

Your PlainDateTime represents "a calendar date and wall-clock time that does not carry time zone information". To convert it to an exact time, you need to supply a timezone, using the toZonedDateTime method. By then ignoring calendar and timezone via the toInstant method, you can get the desired Instant instance.

So there's a few ways to achieve this:

Create a PlainDateTime from an object, convert it to an instant by assuming UTC timezone:

Temporal.PlainDateTime.from({year, month, day, hour, minute}).toZonedDateTime("UTC").toInstant()

Create a PlainDateTime using the constructor, convert it to an instant by assuming UTC timezone:

new Temporal.PlainDateTime(year, month, day, hour, minute).toZonedDateTime("UTC").toInstant()

Create a ZonedDateTime directly from an object, providing the timezone in there, then convert it:
```
Temporal.ZonedDateTime.from({timeZone: 'UTC', year, month, day, hour, minute}).toInstant()
```

Instead of going via a zoned datetime, you can also get the instant that a TimeZone instance ascribes to a PlainDateTime object:

Temporal.TimeZone.from("UTC").getInstantFor(Temporal.PlainDateTime.from({year, month, day, hour, minute}))
new Temporal.TimeZone("UTC").getInstantFor(new Temporal.PlainDateTime(year, month, day, hour, minute))

If you wanted to hardcode the instant in your code, you could also directly create it from an ISO string:
```
Temporal.Instant.from("2022-10-23T02:50Z")
```
If you are open to including the old Date methods, you could also use Date.UTC to compute the millisecond value for the instant - beware zero-based months:
```
Temporal.Instant.fromEpochMilliseconds(Date.UTC(year, month-1, day, hour, minute));
```

Try them for yourselves with your particular example:

const year = 2022;
const month = 10;
const day = 23;
const hour = 2;
const minute = 50;

log(Temporal.PlainDateTime.from({year, month, day, hour, minute}).toZonedDateTime("UTC").toInstant());
log(new Temporal.PlainDateTime(year, month, day, hour, minute).toZonedDateTime("UTC").toInstant());
log(Temporal.ZonedDateTime.from({timeZone: 'UTC', year, month, day, hour, minute}).toInstant());
log(Temporal.TimeZone.from("UTC").getInstantFor(Temporal.PlainDateTime.from({year, month, day, hour, minute})));
log(new Temporal.TimeZone("UTC").getInstantFor(new Temporal.PlainDateTime(year, month, day, hour, minute)));
log(Temporal.Instant.from("2022-10-23T02:50Z"));
log(Temporal.Instant.fromEpochMilliseconds(Date.UTC(year, month-1, day, hour, minute)));

<script src="https://tc39.es/proposal-temporal/docs/playground.js"></script>
<script>function log(instant) { console.log(instant.epochSeconds); }</script>