Potentially not a bug, but the default date for week number is incorrect or suboptimal.

[marktoman]

Creating a date from a week number like this: DateTime.fromObject({ weekYear: 2025, weekNumber: 1 }) resolves to 2024-12-30. This is because the first day of the week falls in the previous year. However, the week belongs to the input date because its Thursday is included in that date.

Wouldn't it make more sense to resolve to a Thursday, then? Doing things like DateTime.fromObject({ weekYear: 2025, weekNumber: 1 }).month would suddenly work.

[diesieben07]

I am not 100% sure I understand your question.
If you specify higher-order units in fromObject but leave out lower-order units then the lower-order units default to their minimum value.
This is what makes DateTime.fromObject({ year: 2025, month: 9 }) resolve to midnight on the 1st of September.

If you're using week units (DateTime.fromObject({ weekYear: 2025, weekNumber: 1 })) then the lower-order units also default to their minimum value, in this case this is weekday, so you get the first day of the week, which is Monday in the ISO week calendar. This is why you get 2024-12-30, because that is the first day of ISO week 1 in ISO week year 2025 (a Monday).
I am unable to follow by what logic a Thursday should appear here, sorry.

Doing things like DateTime.fromObject({ weekYear: 2025, weekNumber: 1 }).month would suddenly work.

Please clarify what you mean by "work". This code works just fine today.

I think this issue might come from the misunderstanding that weekYear and year are the same, when they are not (https://en.wikipedia.org/wiki/ISO_week_date).