A time formatter can format date / time string into a Swift/Obj-c Date object, vice-versa. It uses date format string to determine the format of the date / time string.

Date Format String

For detail of how to wrote the date format string, you can visit NSDateFormatter.com - Easy Skeezy Date Formatting for Swift and Objective-C, it is very helpful.

Below information comes from DateFormatter - Foundation | Apple Developer Documentation official document.

When working with fixed format dates, such as RFC 3339, you set the dateFormat property to specify a format string. For most fixed formats, you should also set the locale property to a POSIX locale (en_US_POSIX), and set the timeZone property to UTC. 

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let RFC3339DateFormatter = DateFormatter()
RFC3339DateFormatter.locale = Locale(identifier: "en_US_POSIX")
RFC3339DateFormatter.dateFormat = "yyyy-MM-dd'T'HH:mm:ssZZZZZ"
RFC3339DateFormatter.timeZone = TimeZone(secondsFromGMT: 0)

/* 39 minutes and 57 seconds after the 16th hour of December 19th, 1996 with an offset of -08:00 from UTC (Pacific Standard Time) */
let string = "1996-12-19T16:39:57-08:00"
let date = RFC3339DateFormatter.date(from: string)

Trouble Shooting

2019-06-10: When you are sure that the date format is correct, but it is return nil after format the date on some devices, please setup a locale.

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let dateFormatter = DateFormatter()
dateFormatter.locale = Locale(identifier: "en_US_POSIX")

Without that, on some device, it will fail to format 24 hours format HH.