Calendar.nextDate mis-computes weekOfYear in certain configurations

Number:rdar://FB12073340 Date Originated:
Status: Resolved:
Product:Foundation Product Version:
Classification:Incorrect/Unexpected Behavior Reproducible:
Consider the following example:

var calendar = Calendar(identifier: .gregorian)
calendar.timeZone = .utc
calendar.firstWeekday = 2

let date = .init(year: 2022)).flatMap {
    calendar.nextDate(after: $0, matching: DateComponents(year: 2023, weekOfYear: 3), matchingPolicy: .strict)

This yields an incorrect value for `date`:
date.ISO8601Format() // INCORRECT: “2023-01-09T00:00:00Z”, SHOULD BE: “2023-01-02T00:00:00Z”

We can illustrate this even further by showing the date components for the date that was calculated:
calendar.dateComponents(Set([.year, .weekOfYear]), from: date) // INCORRECT: “year: 2023 weekOfYear: 3”, SHOULD BE: “year: 2023 weekOfYear: 2”

Even though we asked for a strict date result that has the components year: 2023, weekOfYear: 2, we in fact received a result with a weekOfYear component set to 3.

Unfortunately, this issue carries over into the rest of the year. If we ask for year: 2023, weekOfYear: 5, we receive a date with components year: 2023, weekOfYear: 6.

As a result, every application which relies on this API to calculate the time span of a specific week in this year will be receiving the wrong (subsequent) year under this calendar configuration.


This issue extends to other API as well:

var date = DateComponents(year: 2023))!
date = .weekOfYear, value: 5, of: date)!
calendar.component(.weekOfYear, from: date) // expected “5”, yields “6”

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