Swift 3.0 : Convert server UTC time to local time

问题:

I want to convert server UTC time to local time and vice-versa. Here is my code..

var isTimeFromServer = true
var time:String!
var period:String!
let timeString = "6:59 AM" //Current UTC time

if isTimeFromServer {

    let index = timeString.index(timeString.startIndex, offsetBy: 5)
    let twelve = timeString.substring(to: index)

    var dateString:String!

    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "H:mm"
    let date12 = dateFormatter.date(from: twelve)!

    dateFormatter.dateFormat = "h:mm a"
    let date22 = dateFormatter.string(from: date12)

    //print(date22)
    dateString = date22
    //print("dateString=\(dateString)")

    time = dateString.components(separatedBy: " ")[0]
    period = dateString.components(separatedBy: " ")[1]

}
else {
    time = timeString.components(separatedBy: " ")[0]
    period = timeString.components(separatedBy: " ")[1]
}

var hour = Int(time.components(separatedBy: ":")[0])

hour = period == "AM" ? hour : hour! + 12
let minute = Int(time.components(separatedBy: ":")[1])
let calender = NSCalendar.current
var datecomponent = DateComponents()
datecomponent.calendar = calender
datecomponent.hour = hour
datecomponent.minute = minute

if !isTimeFromServer {
    // local to UTC
    datecomponent.timeZone = TimeZone.current
}
else {
    datecomponent.timeZone = TimeZone(abbreviation: "UTC")
}

let date = datecomponent.date
let dateFormatter = DateFormatter()

if !isTimeFromServer {
    dateFormatter.dateFormat = "H:mm"
    dateFormatter.timeZone = TimeZone(abbreviation: "UTC")
    dateFormatter.string(from: date!)
}
else {
    //UTC to local
    dateFormatter.dateFormat = "h:mm a"
    dateFormatter.timeZone = TimeZone.current
    dateFormatter.string(from: date!)
}

I get the local time

o/p: "12:52 PM"

But actual local time and output time difference is 23 minutes.

回答1:

I don't know what's wrong with your code.
But looks too much unnecessary things are there like you're setting calendar, fetching some elements from string. Here is my small version of UTCToLocal and localToUTC function.
But for that you need to pass string in specific format. Cause I've forcly unwrapped date objects. But you can use some guard conditions to prevent crashing your app.

func localToUTC(date:String) -> String {
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "h:mm a"
    dateFormatter.calendar = NSCalendar.current
    dateFormatter.timeZone = TimeZone.current

    let dt = dateFormatter.date(from: date)
    dateFormatter.timeZone = TimeZone(abbreviation: "UTC")
    dateFormatter.dateFormat = "H:mm:ss"

    return dateFormatter.string(from: dt!)
}

func UTCToLocal(date:String) -> String {
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "H:mm:ss"
    dateFormatter.timeZone = TimeZone(abbreviation: "UTC")

    let dt = dateFormatter.date(from: date)
    dateFormatter.timeZone = TimeZone.current
    dateFormatter.dateFormat = "h:mm a"

    return dateFormatter.string(from: dt!)
}

and call these function like below.

print(UTCToLocal(date: "13:07:00"))
print(localToUTC(date: "06:40 PM"))

Hope this will help you.
Happy coding!!

回答2:

Mrugesh's answer is perfect, but if someone need to use their own formats, or in some different format, I've generalised it so you can give different format or same in both parameters.

func localToUTC(date:String, fromFormat: String, toFormat: String) -> String {

    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = fromFormat
    dateFormatter.calendar = NSCalendar.current
    dateFormatter.timeZone = TimeZone.current
    dateFormatter.date
    let dt = dateFormatter.date(from: date)
    dateFormatter.timeZone = TimeZone(abbreviation: "UTC")
    dateFormatter.dateFormat = toFormat

    return dateFormatter.string(from: dt!)
}

func UTCToLocal(date:String, fromFormat: String, toFormat: String) -> String {
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = fromFormat
    dateFormatter.timeZone = TimeZone(abbreviation: "UTC")

    let dt = dateFormatter.date(from: date)
    dateFormatter.timeZone = TimeZone.current
    dateFormatter.dateFormat = toFormat

return dateFormatter.string(from: dt!)
}

let localDateAsString = UTCToLocal(date: dateAsString!, fromFormat: "hh:mm a, dd MMM yyyy", toFormat: "hh:mm a, dd MMM yyyy")

You can use it as above. Hope it helps.

回答3:

Please try it:

func convertUTCToLocal(timeString: String) -> String? {
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "h:mm a"

    dateFormatter.timeZone = TimeZone.init(abbreviation: "UTC")
    let timeUTC = dateFormatter.date(from: timeString)

    if timeUTC != nil {
        dateFormatter.timeZone = NSTimeZone.local

        let localTime = dateFormatter.string(from: timeUTC!)
        return localTime
    }

    return nil
}



func convertLocalToUTC(localTime: String) -> String? {

    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = "h:mm a"
    dateFormatter.timeZone = NSTimeZone.local
    let timeLocal = dateFormatter.date(from: localTime)

    if timeLocal != nil {
        dateFormatter.timeZone = TimeZone.init(abbreviation: "UTC")

        let timeUTC = dateFormatter.string(from: timeLocal!)
        return timeUTC
    }
    return nil
}


    var isTimeFromServer = true
    var time:String!
    var period:String!
    let timeString = "6:59 AM" //Current UTC time


    if isTimeFromServer {
        print(convertUTCToLocal(timeString: timeString))
    } else {
        print(convertLocalToUTC(localTime: timeString))
    }

回答4:

By the help of Mrugesh Tank Answer,

I have updated his answer and creating the extensions for the date. So that you can easily access the functions from anywhere either from ViewController or either from cell class as well.

extension String {

  //MARK:- Convert UTC To Local Date by passing date formats value
  func UTCToLocal(incomingFormat: String, outGoingFormat: String) -> String {
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = incomingFormat
    dateFormatter.timeZone = TimeZone(abbreviation: "UTC")

    let dt = dateFormatter.date(from: self)
    dateFormatter.timeZone = TimeZone.current
    dateFormatter.dateFormat = outGoingFormat

    return dateFormatter.string(from: dt ?? Date())
  }

  //MARK:- Convert Local To UTC Date by passing date formats value
  func localToUTC(incomingFormat: String, outGoingFormat: String) -> String {
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = incomingFormat
    dateFormatter.calendar = NSCalendar.current
    dateFormatter.timeZone = TimeZone.current

    let dt = dateFormatter.date(from: self)
    dateFormatter.timeZone = TimeZone(abbreviation: "UTC")
    dateFormatter.dateFormat = outGoingFormat

    return dateFormatter.string(from: dt ?? Date())
  }
}

Example how to use it:-

Note:- eventStartDate is the string which you have to converted in your format like this:- "2018-07-11T16:22:00.000Z" 

let finalDate = eventStartDate.UTCToLocal(incomingFormat: "yyyy-MM-dd'T'HH:mm:ss.SSSZ", outGoingFormat: "MMM d, yyyy h:mm a")

回答5:

For everyone using TimeZone objects. I would advise you to create your TimeZone from identifier rather than abbreviation when you have the possibility.

This prevents errors caused by daylight saving.

To illustrate my point let's take an example. You can instantiate like this let timeZone = TimeZone(identifier: "Europe/Paris") or like that let timeZone = TimeZone(abbreviation: "CEST") or "UTC +2:00"

But this is time zone for summer CEST meaning Central Europe Summer Time We have CET meaning Central Europe Time for winter which is "UTC +1:00"

You could manage daylight saving by your own with Date.isDaylightSavingsTime but this means more code and you don't have control on where your daylight saving sprang from. "indicates whether the receiver is currently using daylight saving time" from official doc

All is that is to say favour TimeZone(identifier: ...)