How do I turn an arbitrary time zone into UTC? - swift

Question

I'm allowing my users to be able to select a random Date and time, using their local time zone. I want to be able to send this date string to the server in UTC Format so it can be read by anyone else around the world using their local time zone. I've read lots online on how to turn UTC to local time but not the other way around. How can I accomplish this?
Edit:
First I use the first function to take the concatenated string with a user selected date and time, turn it into a NSDate, and then I convert this NSDate into a UTC string. Is this the best method of achieving my goal?
public class func localTimeZoneStringToDate(string: String) -> NSDate {
let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss"
dateFormatter.timeZone = NSTimeZone.localTimeZone()
return dateFormatter.dateFromString(string)!
}
public class func UTCStringFromDate(date: NSDate) -> String {
let dateFormatter = NSDateFormatter()
dateFormatter.timeZone = NSTimeZone(abbreviation: "UTC")
dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss"
return dateFormatter.stringFromDate(date)
}

Answer 1

If you're using the UIDatePicker, getting the selected date and time in UTC is quite simple. The first line of code below will return the selected time in YYYY-MM-DD hh:mm:ss The second line of code will return the time interval in seconds since January 1, 1970 at 12:00am GMT.
// Returns Selecteed Date //
datePicker.date
// Returns Seconds Since Jan. 1, 1970 //
datePicker.date.timeIntervalSince1970
However, if you are not using the UIDatePicker, you can get the same information that the above code will return by simply using:
//** Variable "pastDate" must be NSDate **//
// Returns Selected Date //
pastDate
// Returns Seconds Since Jan.1, 1970 //
pastDate.timeIntervalSince1970
EDIT:
I think I understand what you're attempting to do now, and the following code should return the user's selected date in UTC form.
func UTCStringFromDate(date: NSDate) -> String {
// Get User's Time //
let calendar = NSCalendar.currentCalendar()
// Get User's TimeZone Difference //
let difference = calendar.timeZone.secondsFromGMT
// Get UTC Time //
let adjustedTime = calendar.dateByAddingUnit(.Second, value: -difference, toDate: date, options: [])!
// Format Date //
let dateFormatter = NSDateFormatter()
dateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ss"
return dateFormatter.stringFromDate(adjustedTime)
}