Xcode – DateFormatter is returning nil date for a simple date string

I’m having an issue where an attempt to turn a string into a date via a DateFromatter has produced nothing but ‘nil’ and I don’t know where I’m going wrong. The code is as simple as can be:

    let testDate = "2021"

    let formatter = DateFormatter()
    formatter.locale = Locale(identifier: "en_US_POSIX")
    formatter.dateFormat = "yyyy"

    formatter.timeZone = TimeZone(secondsFromGMT: 0)


    let date = formatter.date(from: testDate)

Note that this is much simpler than originally, the date I’m trying to format is actually:

"2021-05-01T01:00:00Z"

But I’ve stripped it right down to narrow down where the issue is.

As you can see from above, I’ve stripped down to a year, configured the DateFormatter with en_US_POSIX and used only the ‘yyyy’ as a format. This works in Playgrounds, but it doesn’t work in my Xcode simulator (Which is in the US locale) or my own physical iPhone (set to UK locale). That being said, I’ve no idea why the locale would matter because the string in question is just a year – it’m not even getting a wrong year, just nil.

I’m not sure where I’m going wrong.