前一天的日期是凌晨4点

Question

鉴于日期，我想在4:00收到日期。这在0:00 - 3:39失败：

let date3am = Calendar.current.date(from: DateComponents(year: 2017, month: 7, day: 1, hour: 3, minute: 0, second: 0))!

let date = Calendar.current.date(bySettingHour: 4, minute: 0, second: 0, of: 
date3am, direction: .backward)!

date // "Jul 1, 2017, 4:00 AM"
date < date3am // false

为什么date(bySettingHour)不会倒退？

Answer 1

您使用的是错误的方法。您需要使用日历方法nextDate(after date:)，如下所示：

let date3am = Calendar.current.date(from: DateComponents(year: 2017, month: 7, day: 1, hour: 3, minute: 0, second: 0))!

if let date = Calendar.current.nextDate(after: date3am, matching: DateComponents(hour: 4, minute: 0, second: 0), matchingPolicy: .nextTime, repeatedTimePolicy: .first, direction: .backward) {
    date // "Jun 30, 2017, 4:00 AM"
    date < date3am // true
}