Swift：timeIntervalSince1970 的精度

Question

当我将 Unix 纪元时间转换为毫秒时，它会四舍五入。

代码：

import Foundation

extension Date {
    public func getMilliSeconds()-> TimeInterval {
        return TimeInterval(self.timeIntervalSince1970)
    }
}

let epochTime: TimeInterval = 1597269862.9328
print("\(epochTime) Precise Epoch Time /// (notice the 8 is the last digit)")

let convertedTime = Date(timeIntervalSince1970: epochTime)
print("\(convertedTime) Time converted To Swift Date")

let df = DateFormatter()
df.dateFormat = "y-MM-dd H:m:ss.SSSS"
let withMilli = df.string(from: convertedTime)
print("\(withMilli) Time with milliseconds using Date Formatter /// (notice 8 is missing and this was rounded to .9330")

if let convertedBackToEpochTime = df.date(from: withMilli) {
    print("\(convertedBackToEpochTime.getMilliSeconds()) Time converted back to Unix epoch time")
}

日志：

1597269862.9328 Precise Epoch Time /// (notice the 8 is the last digit)
2020-08-12 22:04:22 +0000 Time converted To Swift Date
2020-08-12 15:4:22.9330 Time with milliseconds using Date Formatter /// (notice 8 is missing and this was rounded to .9330
1597269862.933 Time converted back to Unix epoch time

我以 1597269862.9328 开头，以 1597269862.933 结尾。我怎样才能使日期准确地回到我开始时的日期：1597269862.9328?

Answer 1

日期的所有组成部分（年、月、日、时、分、秒等）都是整数。毫秒，顾名思义，只精确到 one-thousandth 秒（即：3 位小数）。

你要得到的是nanosecond属性。这是一个使用 Swift 5.0 中添加的自定义插值功能的示例：

import Foundation

extension String.StringInterpolation {
    /// Our custom interpolator for Date
    mutating func appendInterpolation(custom date: Date) {
        let components: Set<Calendar.Component> = [
            .year, .month, .day, .hour, .minute, .second, .nanosecond, .timeZone
        ]
        let values = Calendar.current.dateComponents(components, from: date)
        appendLiteral(values.description)
    }
}

let epochTime: TimeInterval = 1597269862.9328
print("\(epochTime) Precise Epoch Time (notice the 8 is the last digit)")

let convertedTime = Date(timeIntervalSince1970: epochTime)
print("\(custom: convertedTime)")

结果：

1597269862.9328 Precise Epoch Time (notice the 8 is the last digit)
timeZone: America/Toronto (current) year: 2020 month: 8 day: 12 hour: 18 minute: 4 second: 22 nanosecond: 932800054 isLeapMonth: false

通知nanosecond: 932800054。此值与原始输入的差异是由于浮点精度，这是不可避免的。

Answer 2

问题是 DateFormatter 限制在毫秒内。使用它无法显示超过 3 个小数位。

extension Formatter {
    static let iso8601withFractionalSeconds: DateFormatter = {
        let formatter = DateFormatter()
        formatter.calendar = Calendar(identifier: .iso8601)
        formatter.locale = Locale(identifier: "en_US_POSIX")
        formatter.timeZone = TimeZone(secondsFromGMT: 0)
        formatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss.SSSSSSSSSXXXXX"
        return formatter
    }()
}

Formatter.iso8601withFractionalSeconds.string(from: Date())  // "2020-10-29T16:12:27.111000000Z"

如果您想取回初始值，您只需要从转换时间日期获取 timeIntervalSince1970：

let epochTime: TimeInterval = 1597269862.9328
print("\(epochTime) Precise Epoch Time")

let convertedTime = Date(timeIntervalSince1970: epochTime)
print("\(convertedTime) Time converted To Swift Date")

print(convertedTime.timeIntervalSince1970)   // "1597269862.9328\n"

请注意，Swift Date 存储为自参考日期以来的时间间隔。如果您想保持日期的准确性，您应该在存档时使用 timeIntervalSinceReferenceDate 而不是 timeIntervalSince1970。检查这个 .

Swift：timeIntervalSince1970 的精度

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