Adding Time Offset in Swift

Question

I have a bunch of different show times in a database and want to display the correct time based on the users time zone by creating an offset.

I\'m getting the users

Answer 1

If you want to convert a show time which is stored as a string in GMT, and you want to show it in the user's local time zone, you should not be manually adjusting the NSDate/Date objects. You should be simply using the appropriate time zones with the formatter. For example, in Swift 3:

let gmtTimeString = "5:00 PM"

let formatter = DateFormatter()
formatter.dateFormat = "h:mm a"
formatter.timeZone = TimeZone(secondsFromGMT: 0)            // original string in GMT
guard let date = formatter.date(from: gmtTimeString) else {
    print("can't convert time string")
    return
}

formatter.timeZone = TimeZone.current                       // go back to user's timezone
let localTimeString = formatter.string(from: date)

Or in Swift 2:

let formatter = NSDateFormatter()
formatter.dateFormat = "h:mm a"
formatter.timeZone = NSTimeZone(forSecondsFromGMT: 0)  // original string in GMT
let date = formatter.dateFromString(gmtTimeString)

formatter.timeZone = NSTimeZone.localTimeZone()        // go back to user's timezone
let localTimeString = formatter.stringFromDate(date!)

Answer 2

I would use the dateByAddingTimeInterval function to add and subtract hours. Add a to the dateFormat string to print am or pm.

var showTimeStr = "00:00 PM" //show time in GMT as String
let formatter = NSDateFormatter()
formatter.dateFormat = "hh:mm a"
let showTime = formatter.dateFromString(showTimeStr)
showTime.dateByAddingTimeInterval(3600) //Add number of hours in seconds, subtract to take away time
showTimeStr = formatter.stringFromDate(showTime)

Answer 3

Here is my final code, combined with b.Morgans answer. I believe it's all working now.

let offsetTime = NSTimeInterval(NSTimeZone.localTimeZone().secondsFromGMT)

var showTimeStr = "05:00 PM" //show time in GMT as String
let formatter = NSDateFormatter()
formatter.dateFormat = "h:mm a"
let showTime = formatter.dateFromString(showTimeStr)
let finalTime = showTime?.dateByAddingTimeInterval(offsetTime) //Add number of hours in seconds, subtract to take away time
showTimeStr = formatter.stringFromDate(finalTime!)