Dates and times independent of time zone.
You probably don't need this! It should be very rare to need to a date independent of time zone. When in doubt, just use Apple's Date type.
Apple's Date type is independent of time zone, but it represents a specific point in time. By contrast, an AbsoluteDate represents not a specific point in time, but a "floating" date such as "2018-01-20 18:30:33". The actual point in time represented by "2018-01-20 18:30:33" depends upon which time zone it's in.
Most of the time you should not need this. But there are rare cases where it can be useful. It is particularly useful when using AbsoluteDay, which represents a day independent of time and time zone.
Imagine an application that tracks various activities a user performs. This user is a globetrotter and is frequently in different time zones. These activities are merged together at the end of the day to produce an "activity score". But which day do we use? Moving across time zones, particularly in the middle of the night, can produce changes in which day it is. In this case, we'll use the day as the user perceives it, the AbsoluteDay. By passing a Date and TimeZone to the initializer of AbsoluteDay, we strip out those considerations and just use an abstract "day", which we then use to perform our activity score calculation.
You could. But the types contained in this framework offer some conveniences over a string. For instance, given a variable d of type AbsoluteDay, we can say d + 1 to get the next AbsoluteDay. You can also use the date property or the date function to convert back to a Date very easily.
All of the types contained herein serialize using Apple's Codable. While they always encode to a String, they can decode from a Date or String. How a Date decodes is dependent upon the dateDecodingStrategy property of the Decoder. Once the Date is decoded, the AbsoluteDate is derived by formatting it in the current time zone. To use a different time zone, set the CodingUserInfoKey.absoluteDateTimeZone key of the decoder's userInfo property to your preferred time zone, e.g.,
let decoder = JSONDecoder()
decoder.userInfo[.absoluteDateTimeZone] = TimeZone(identifier: "UTC")!
let absoluteDate = try decoder.decode(AbsoluteDate.self, from: jsonData)