DateFormatter ISO8601 TimeZones

Question

I'm attempting to add parsing validation tests and wanted to check that the initial JSON I was sent could be turned into an object and that object in turn turned into JSON. In the end the validation would be that both dictionaries are equal. What I'm seeing however is that, while date parsing works, the conversion to a string replaces +00:00 with Z . In my research I've found that these are interchangeable and I'm aware that I could in theory replace Z with +00:00 for the comparison but I was wondering if there is a way on the ISO8601DateFormatter or any DateFormatter to say that you would prefer +00:00 over Z ?

For those who like to see some code this is my quick playground example.

var date = "2018-01-30T22:13:12+00:00"
let df = ISO8601DateFormatter()
df.formatOptions = [.withInternetDateTime]

let newDate = df.date(from: date)
let newString = df.string(from: newDate!)

Answer 1

The ISO 8601 date format states that Z should be used when the date's timezone offset is 0. Many of the timezone date formatting symbols used with a DateFormatter also specifically result in Z if the date's timezone offset is 0.

If you want to generate a string from a Date and you want to ensure that you get +00:00 instead of Z , then use DateFormatter with the appropriate date formatter specifier.

The format specifier xxx will give you a timezone in the format +00:00 . XXX and ZZZZZ will also give you that same format but will give you Z in the result if the offset is 0. More on these can be seen on the Unicode Technical Specification #35 page.

The documentation for ISO8601DateFormatter and its formatOptions states that ZZZZZ is used for the timezone. So you will always get Z for a timezone offset of 0.

A DateFormatter with a date format of yyyy-MM-dd'T'HH:mm:ssxxx will give you the same result you are looking for. But also be sure to set the date formatter's locale to en_US_POSIX . You will also need to ensure the output comes out in the UTC timezone. Set the formatter's timeZone property to TimeZone(secondsFromGMT: 0) .