问题

I want to check if the current date input is 1 second away from new date (Date format: "2023-05-20T23:59:59.000+05:30"). Below is my Date extension.

extension Date {
    var isOneSecondAway: Bool {
        let calendar = Calendar.current
        let nextStartOfDay = calendar.date(byAdding: .day, value: 1, to: self.startOfDay) ?? self
        let timeIntervalInSeconds = Int(nextStartOfDay.startOfDay.timeIntervalSince(self))
        return timeIntervalInSeconds == 1
    }
    
    var startOfDay: Date {
        return Calendar.current.startOfDay(for: self)
    }
}

let dateFormatter = DateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSZZZZZ"
let targetDateString = "2023-05-20T23:59:59.000+05:30"
guard let targetDate = dateFormatter.date(from: targetDateString) else {return}
print(targetDate.isOneSecondAway)

Expected output for "2023-05-20T23:59:59.000+05:30" is true but it's returning false. Am I doing something wrong here?

<details>
<summary>英文:</summary>

I want to check if the current date input is 1 second away from new date (Date format: &quot;2023-05-20T23:59:59.000+05:30&quot;). Below is my Date extension.

    extension Date {
        var isOneSecondAway: Bool {
            let calendar = Calendar.current
            let nextStartOfDay = calendar.date(byAdding: .day, value: 1, to: self.startOfDay) ?? self
            let timeIntervalInSeconds = Int(nextStartOfDay.startOfDay.timeIntervalSince(self))
            return timeIntervalInSeconds == 1
        }
        
        var startOfDay: Date {
            return Calendar.current.startOfDay(for: self)
        }
    }
    
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = &quot;yyyy-MM-dd&#39;T&#39;HH:mm:ss.SSSZZZZZ&quot;
    let targetDateString = &quot;2023-05-20T23:59:59.000+05:30&quot;
    guard let targetDate = dateFormatter.date(from: targetDateString) else {return}
    print(targetDate.isOneSecondAway)

Expected output for &quot;2023-05-20T23:59:59.000+05:30&quot; is true but its returning false. Am I doing something wrong here?


</details>


# 答案1
**得分**: 1

You need a timezone in order to figure out when the "start of day" is, and to a lesser extent, when the "next day" is.

You used `Calendar.current` in your `Date` extensions, so the timezone of `Calendar.current` will be used to do the date calculations. This is usually the system timezone. Note that a `Date` object does not have an associated timezone. `Calendar` does.

I suspect that the timezone of `Calendar.current` *isn't* at an offset of +05:30 for the date and time that you used.

If you set the timezone explicitly to be at an offset of +05:30, then the code prints `true`:

```swift
extension Date {
    var isOneSecondAway: Bool {
        var calendar = Calendar.current
        calendar.timeZone = TimeZone(secondsFromGMT: 5 * 60 * 60 + 30 * 60)!
        let nextStartOfDay = calendar.date(byAdding: .day, value: 1, to: self.startOfDay) ?? self
        let timeIntervalInSeconds = Int(nextStartOfDay.startOfDay.timeIntervalSince(self))
        return timeIntervalInSeconds == 1
    }
    
    var startOfDay: Date {
        var calendar = Calendar.current
        calendar.timeZone = TimeZone(secondsFromGMT: 5 * 60 * 60 + 30 * 60)!
        return calendar.startOfDay(for: self)
    }
}

Of course, this wouldn't be very useful if your date strings can contain timezones other than +05:30.

To handle all kinds of timezones, the Date extensions need a timezone parameter, and you need to extract the timezone from the strings.

extension Date {
    func isOneSecondAway(timeZone: TimeZone) -> Bool {
        var calendar = Calendar.current
        calendar.timeZone = timeZone
        let nextStartOfDay = calendar.date(byAdding: .day, value: 1, to: self.startOfDay(in: timeZone)) ?? self
        let timeIntervalInSeconds = Int(nextStartOfDay.startOfDay(in: timeZone).timeIntervalSince(self))
        return timeIntervalInSeconds == 1
    }
    
    func startOfDay(in timeZone: TimeZone) -> Date {
        var calendar = Calendar.current
        calendar.timeZone = timeZone
        return calendar.startOfDay(for: self)
    }
}

// ...
if let targetDate = dateFormatter.date(from: targetDateString),
   let timeZone = TimeZone(iso8601: targetDateString) {
    print(targetDate.isOneSecondAway(timeZone: timeZone))
}

Here I used a TimeZone initializer, adapted from this post.

extension TimeZone {
    init?(iso8601: String) {
        // assuming there are 23 characters before the offset,
        // like your format string suggests
        let tz = iso8601.dropFirst(23) // remove date and time part
        if tz == "Z" {
            self.init(secondsFromGMT: 0)
        } else if tz.count == 3 { // assume +/-HH
            if let hour = Int(tz) {
                self.init(secondsFromGMT: hour * 3600)
                return
            }
        } else if tz.count == 5 { // assume +/-HHMM
            if let hour = Int(tz.dropLast(2)), let min = Int(tz.dropFirst(3)) {
                self.init(secondsFromGMT: (hour * 60 + min) * 60)
                return
            }
        } else if tz.count == 6 { // assume +/-HH:MM
            let parts = tz.components(separatedBy: ":")
            if parts.count == 2 {
                if let hour = Int(parts[0]), let min = Int(parts[1]) {
                    self.init(secondsFromGMT: (hour * 60 + min) * 60)
                    return
                }
            }
        }

        return nil
    }
}

extension Date {
var isOneSecondAway: Bool {
var calendar = Calendar.current
calendar.timeZone = TimeZone(secondsFromGMT: 5 * 60 * 60 + 30 * 60)!
let nextStartOfDay = calendar.date(byAdding: .day, value: 1, to: self.startOfDay) ?? self
let timeIntervalInSeconds = Int(nextStartOfDay.startOfDay.timeIntervalSince(self))
return timeIntervalInSeconds == 1
}
var startOfDay: Date {
var calendar = Calendar.current
calendar.timeZone = TimeZone(secondsFromGMT: 5 * 60 * 60 + 30 * 60)!
return calendar.startOfDay(for: self)
}
}

extension Date {
func isOneSecondAway(timeZone: TimeZone) -> Bool {
var calendar = Calendar.current
calendar.timeZone = timeZone
let nextStartOfDay = calendar.date(byAdding: .day, value: 1, to: self.startOfDay(in: timeZone)) ?? self
let timeIntervalInSeconds = Int(nextStartOfDay.startOfDay(in: timeZone).timeIntervalSince(self))
return timeIntervalInSeconds == 1
}
func startOfDay(in timeZone: TimeZone) -> Date {
var calendar = Calendar.current
calendar.timeZone = timeZone
return calendar.startOfDay(for: self)
}
}
// ...
if let targetDate = dateFormatter.date(from: targetDateString),
let timeZone = TimeZone(iso8601: targetDateString) {
print(targetDate.isOneSecondAway(timeZone: timeZone))
}

extension TimeZone {
init?(iso8601: String) {
// assuming there are 23 characters before the offset,
// like your format string suggests
let tz = iso8601.dropFirst(23) // remove date and time part
if tz == "Z" {
self.init(secondsFromGMT: 0)
} else if tz.count == 3 { // assume +/-HH
if let hour = Int(tz) {
self.init(secondsFromGMT: hour * 3600)
return
}
} else if tz.count == 5 { // assume +/-HHMM
if let hour = Int(tz.dropLast(2)), let min = Int(tz.dropFirst(3)) {
self.init(secondsFromGMT: (hour * 60 + min) * 60)
return
}
} else if tz.count == 6 { // assime +/-HH:MM
let parts = tz.components(separatedBy: ":")
if parts.count == 2 {
if let hour = Int(parts[0]), let min = Int(parts[1]) {
self.init(secondsFromGMT: (hour * 60 + min) * 60)
return
}
}
}
return nil
}
}