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JS TutorialLesson #212

JavaScript Temporal Mistakes

Temporal is a powerful API, but it can be confusing at first.

Remember to use the correct Temporal type
Avoid implicit conversions
Always handle time zones correctly
Use compare() instead of
Remember that Temporal Objects are immutable

Here are some common mistakes and how to avoid them.

Missing UTC (Z) for Instant

An Instant must always include UTC.

Wrong

Example Code

javascript

Try it Yourself

const instant = Temporal.Instant.from("2026-05-17T14:30:00");

Using PlainDateTime with Time Zone

PlainDateTime does not support time zones.

Wrong

Example Code

javascript

Try it Yourself

const date = Temporal.PlainDateTime.from("2026-05-17T14:30:00Z");

Comparing Different Types

Temporals can go wrong when comparing different types.

Wrong

Example Code

javascript

Try it Yourself

const d1 = Temporal.PlainDate.from("2026-05-17");

const d2 = Temporal.Instant.from("2026-05-17T14:30Z");

Temporal.PlainDate.compare(d1, d2);

Correct

Example Code

javascript

Try it Yourself

const d1 = Temporal.PlainDate.from("2026-05-17");

const d2 = Temporal.PlainDateTime.from("2026-05-17T14:30");

Temporal.PlainDate.compare(d1, d2);

Using equals() with Different Types

Temporals can go wrong when comparing different types.

Wrong

Example Code

javascript

Try it Yourself

const d1 = Temporal.PlainDate.from("2026-05-17");

const d2 = Temporal.Instant.from("2026-05-17T14:30Z");

d1.equals(d2);

Correct

Example Code

javascript

Try it Yourself

const d1 = Temporal.PlainDate.from("2026-05-17");

const d2 = Temporal.PlainDateTime.from("2026-05-17T14:30");

d1.equals(d2.toPlainDate());

Using ==, for Comparison

Temporal objects cannot be compared with ==, ===, .

Wrong

Example Code

javascript

Try it Yourself

const d1 = Temporal.PlainDate.from("2026-05-17");

const d2 = Temporal.PlainDate.from("2026-05-17");

d1 === d2; // False

d1 == d2;  // False

d1   d2;   // TypeError

Expecting Mutation

Temporal objects are immutable.

Wrong

Example Code

javascript

Try it Yourself

const date = Temporal.PlainDate.from("2026-05-17");

date.add({ months: 1 });

Using valueOf()

Temporal objects do not convert to numbers.

Example

Example Code

javascript

Try it Yourself

const date = Temporal.PlainDate.from("2026-05-17");

try {

  text = date.valueOf();

} catch (err) {

  text = err.name;

}

Using Instant for Local Time

Instant is always UTC (not local time).

Wrong

Example Code

javascript

Try it Yourself

Temporal.Now.instant(); // not local time

Using PlainDate for Time

PlainDate has no time.

Wrong

Example Code

javascript

Try it Yourself

Temporal.PlainDate.from("2026-05-17T14:30");

Forgetting Time Zone in ZonedDateTime

ZonedDateTime requires a time zone.

Wrong

Example Code

javascript

Try it Yourself

Temporal.ZonedDateTime.from("2026-05-17T14:30:00");

Correct

Example Code

javascript

Try it Yourself

Temporal.ZonedDateTime.from("2026-05-17T14:30:00+02:00[Europe/Oslo]");

Not Choosing the Right Type

Each Temporal type has a specific purpose.

Type	Use
Instant	Exact moment (UTC)
PlainDate	Date only
PlainTime	Time only
PlainDateTime	Date + time only
ZonedDateTime	Date + time + time zone
Duration	Length of time only