Is it expected that dt.set() will never return invalid dates?

[hashhar]

For example:

const today = DateTime.now().startOf("day");

let dt = DateTime.fromISO("2004-02-29");
let next = dt.set({ year: today.year });
console.log(next.isValid); -- returns true and value is 2025-02-28

next = dt.set({ year: 2028 });
console.log(next.isValid); -- returns true and value is 2028-02-29

i.e. in the first invocation it didn't generate 2025-02-29 which would be an invalid date. It seems some fallback was applied internally.

Is this expected or am I relying on some undocumented bug/behaviour of Luxon by taking advantage of this behaviour?

[diesieben07]

This is expected, invalid dates are only produced if the values you're providing are completely invalid.
For "day of the month" specifically Luxon will clamp it to the valid month range, if you're not the one setting it, as in your example.
In general, set and fromObject will behave like Date.UTC in that units that are out of range will overflow to the others.

[dobon]

Thanks for the report. You're right that a fallback is used internally to preserve validity (changing the date from 29 to 28 for non-leap years). That is expected behavior and you can take advantage of it safely.

That being said, I can see why it might feel strange and unintuitive that

DateTime.fromISO("2005-02-29").set({year: 2004}).isValid
>false
DateTime.fromISO("2004-02-29").set({year: 2005}).isValid
>true

[diesieben07]

That being said, it does feel strange and unintuitive that

DateTime.fromISO("2005-02-29").set({year: 2004}).isValid
>false
DateTime.fromISO("2004-02-29").set({year: 2005}).isValid
>true

Not strange at all, the invalid date in the 2nd example is 2004-02-29, the set has no effect at all.

[dobon]

I agree that's the correct behavior. My phrasing was a bit jumbled, but I meant to express that it could be confusing to people who are new to Luxon and making assumptions. If you were to break up the chained method invocations in my snippet onto separate lines then the fact that set has no effect on an invalid DateTime is clear.