从weekOfYear获取不正确的开始和结束日期

Question

let calendar = Calendar.current
    var dateComponents = DateComponents()
    dateComponents.weekday = calendar.firstWeekday
    dateComponents.weekOfYear = 2
    dateComponents.year = 2017
    let startOfWeek = calendar.date(from: dateComponents)
    let endOfWeek = calendar.date(byAdding: .day, value: 6, to: startOfWeek!)
    let formatter = DateFormatter()
    formatter.dateStyle = .short
    print(formatter.string(from: startOfWeek!))
    print(formatter.string(from: endOfWeek!))

打印:

1/8/17
    1/14/17

改变代码:

dateComponents.weekOfYear = 1
    dateComponents.year = 2017

打印:

12/31/17
    1/6/18

为什么是12/31/17？

有谁告诉我？非常感谢.

Answer 1

当我使用.full样式打印日期时,我会在2017年12月31日星期日的第一个日期,但这显然是错误的,因为12月31日是星期四.

如果您想获得正确的日期,请使用yearForWeekOfYear而不是year.文件:

您可以将yearForWeekOfYear属性与weekOfYear和weekday属性一起使用,以获取与一年中给定周的特定工作日相对应的日期.例如,2005年第53周的第6天(ISO 2005-W53-6)对应于格里高利历的2005年1月1日星期六.

另外,你可能有点顽皮而不听文件并使用weekOfYear = 54:

let calendar = Calendar.current
var dateComponents = DateComponents()
dateComponents.weekday = calendar.firstWeekday
dateComponents.weekOfYear = 54
dateComponents.year = 2017
let startOfWeek = calendar.date(from: dateComponents)
let endOfWeek = calendar.date(byAdding: .day, value: 6, to: startOfWeek!)
let formatter = DateFormatter()
formatter.dateStyle = .short
print(formatter.string(from: startOfWeek!))
print(formatter.string(from: endOfWeek!))

这打印:

1/1/17
1/7/17

巧合的是,正确的日期.